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Table salt may contain additional unwanted ingredients. Only pure NaCl or minerals found in sea water should go into such a solution. But food industry is usually allowed to add much more.

Nasal irrigation salt needs just to be NaCl used in isotonic concentration. The indian tradition is ancient and based on water from rivers, which is not isotonic and now often contaminated.

Clean (i.e. lower salt concentration; but if the water supply isn't safe also meaning: sterilised?), potable/drinking tap-water works as well, but appears much harsher than needed. It is uncomfortable. So isotonic solutions are clearly preferable. Too low salt concentrations let the cells swell up, too high a concentration dries them out and will lead in the mid term to much worse results. The added salt is not needed for the practice , but beneficial if the concentration is hovering around 0.9%.

One can use regular salt for that. As long as it's pure NaCl. And one may be 'lucky' in using all other forms of table salt. But regular commercially sold table salt is usually not just NaCl!

First link to vendor appearing in a websearch:

Whether sea salts minerals are in there as well is not really that much more beneficial, but when it's regular "table salt" then most jurisdiction allow a lot of other ingredients in it that you do not want in your nose pr on your mucus membrane.

Regular kitchen table salt can be used but is not recommended because of all the additives.

Apart from possible adulterations when using "funny" salts ('Himalaya' etc)  allowed anti-caking agents for example are:

Are additives allowed in the salt I use in my organic products? Though salt and sea salt are allowed in all categories of organic products, anti-caking/free-flow agents and whiteners are generally not allowed in these products. Common anti-caking/free-flow agents that are not allowed include:

In Europe, only E170 Calcium carbonate is allowed in salt for organic uses.

Ordinary table salts may still contain (sodium and magnesium carbonate (E 500, E 504), sodium-, potassium- and calcium ferrocyanides (E 535, E 536, E 538), silicon dioxide, calcium- and magnesium silicates (E 551 – E 553), iron tartrate (E 534). Aluminium salts are no longer allowed in Europe as an additive, but can be a found in some salts.

Further regular nasal irrigation isn't very well supported for most use case anyway:

Karen Head, Kornkiat Snidvongs, Simon Glew, Glenis Scadding, Anne GM Schilder, Carl Philpott, Claire Hopkins: "Saline irrigation for allergic rhinitis", in: Cochrane Database of Systematic Reviews. No. 6., 22. June 2018. doi:10.1002/14651858.CD012597.pub2

Saline nasal irrigation: Its role as an adjunct treatment

Disclaimer:
ill people or those with recent surgery as well as extra paranoid germaphobes might want to boil the water before use, to kill any of the germs potentially present in any water, then wait until it reaches body temperature.
If the municipal water supply is reasonably clean this is not necessary for healthy adults. People in third world countries like the USA may want boil the water for all the reasons above anyway. In regions with sane safety standards regarding the water supply that are actually enforced, many doctors often explicitly deny the need to boil water first.

Main sources for bacterial contamination are preparing a too large portion of solution and letting it sit and the pots used themselves:

Although common and frequently based on potentially dangerous bacteria, contamination is considered a false problem by some experts. They think that the nasal cavity is naturally full of bacteria and the addition of new pathogens is not clinically relevant.

The problem of sterility of the solutions and devices has been debated.  Solutions are at risk of contamination when large volumes of solution based on distilled water, bottled water, or boiled water are prepared at home, maintained in containers and used each time when NI is needed by withdrawing the required amount of liquid. Devices can be contaminated when they are continuously used without adequate cleaning. Lee et al. reported that after one and two weeks of use, irrigation bottles used by adults undergoing endoscopic sinus surgery that were washed with hot soapy water after each use were found to be contaminated by a large spectrum of bacteria, including Pseudomonas aeruginosa, Serratia marcescens, Proteus mirabilis, and Staphylococcus aureus. Similar findings were reported by other authors and, because in many cases contaminating bacteria were the same as those that could cause acute rhinosinusitis, it was suggested that the main source of device colonization was the sinonasal cavities. The risk of contamination seems independent of the type of device. Additionally, the use of a one-way valve irrigation bottle, theoretically capable of reducing the risk of reflux of contaminated solution in the device, was found to be practically ineffective. In contrast, contamination seems to be influenced by the composition of the solution. It was shown that acidic, isotonic saline solutions were more frequently associated with bacterial contamination probably because some of the most common contaminants grow optimally in similar environmental conditions. Finally, contamination was found more frequently with longer durations of NI use. With some exceptions, studies have reported that both bottles and bulb syringes were contaminated after one to two weeks of use in approximately 25% of the cases and in 45% after four weeks.

Table salt contains additional unwanted ingredients.

First link to vendor appearing in a websearch selling overpriced salt says:

That is not to say that vendors opting for the name 'neti' aren't willing to also add 'exotic' stuff, that's then not beneficial either.

Regular kitchen table salt can be used but will cause problems because of all the additives.

Apart from possible unknown adulterations when using "funny" salts ('Himalaya' etc) just the usually allowed anti-caking agents for example are:

Common anti-caking/free-flow agents that are allowed in the US table salt include:

Ordinary table salts may still contain (sodium and magnesium carbonate (E 500, E 504), sodium-, potassium- and calcium ferrocyanides (E 535, E 536, E 538), silicon dioxide, calcium- and magnesium silicates (E 551 – E 553), iron tartrate (E 534). Aluminium salts are officially no longer allowed in Europe as an additive, but can be a found in some salts.

Main sources for bacterial contamination are neither the salts nor the water. Preparing a too large portion of solution in advance and letting it sit, and the pots used themselves are:

Although common and frequently based on potentially dangerous bacteria, contamination is considered a false problem by some experts. They think that the nasal cavity is naturally full of bacteria and the addition of new pathogens is not clinically relevant.

The problem of sterility of the solutions and devices has been debated. 

Solutions are at risk of contamination when large volumes of solution based on distilled water, bottled water, or boiled water are prepared at home, maintained in containers and used each time when NI is needed by withdrawing the required amount of liquid. Devices can be contaminated when they are continuously used without adequate cleaning. Lee et al. reported that after one and two weeks of use, irrigation bottles used by adults undergoing endoscopic sinus surgery that were washed with hot soapy water after each use were found to be contaminated by a large spectrum of bacteria, including Pseudomonas aeruginosa, Serratia marcescens, Proteus mirabilis, and Staphylococcus aureus. Similar findings were reported by other authors and, because in many cases contaminating bacteria were the same as those that could cause acute rhinosinusitis, it was suggested that the main source of device colonization was the sinonasal cavities. The risk of contamination seems independent of the type of device. Additionally, the use of a one-way valve irrigation bottle, theoretically capable of reducing the risk of reflux of contaminated solution in the device, was found to be practically ineffective. In contrast, contamination seems to be influenced by the composition of the solution. It was shown that acidic, isotonic saline solutions were more frequently associated with bacterial contamination probably because some of the most common contaminants grow optimally in similar environmental conditions. Finally, contamination was found more frequently with longer durations of NI use. With some exceptions, studies have reported that both bottles and bulb syringes were contaminated after one to two weeks of use in approximately 25% of the cases and in 45% after four weeks.

Nasal irrigation salt needs just to be NaCl used in isotonic concentration. The indian tradition is based on water from rivers, which is not isotonic and now often contaminated.

Clean (i.e. lower salt concentration; but if the water supply isn't safe also meaning: sterilised?), potable/drinking tap-water works as well, but appears much harsher than needed. It is uncomfortable. So isotonic solutions are clearly preferable. Too low salt concentrations let the cells swell up, too high a concentration dries them out and will lead in the mid term to much worse results. The added salt is not needed for the practuce , but beneficial if the concentration is hovering around 0.9%.

Disclaimer: ill people or those with recent surgery as well as extra paranoid germaphobes might want to boil the water before use, to kill any of the germs potentially present in any water, then wait until it reaches body temperature. If the municipal water supply is reasonably clean this is not necessary for healthy adults. People in third world countries like the USA may want boil the water for all the reasons above anyway.

Nasal irrigation salt needs just to be NaCl used in isotonic concentration. The indian tradition is ancient and based on water from rivers, which is not isotonic and now often contaminated.

Clean (i.e. lower salt concentration; but if the water supply isn't safe also meaning: sterilised?), potable/drinking tap-water works as well, but appears much harsher than needed. It is uncomfortable. So isotonic solutions are clearly preferable. Too low salt concentrations let the cells swell up, too high a concentration dries them out and will lead in the mid term to much worse results. The added salt is not needed for the practice , but beneficial if the concentration is hovering around 0.9%.

Disclaimer:
ill people or those with recent surgery as well as extra paranoid germaphobes might want to boil the water before use, to kill any of the germs potentially present in any water, then wait until it reaches body temperature.
If the municipal water supply is reasonably clean this is not necessary for healthy adults. People in third world countries like the USA may want boil the water for all the reasons above anyway. In regions with sane safety standards regarding the water supply that are actually enforced, many doctors often explicitly deny the need to boil water first.

Nasal irrigation salt needs just to be NaCl used in isotonic concentration. The indian tradition is based on water from rivers, which is not isotonic and now often contaminated. Clean (sterilised?), potable/drinking tap-water works as well, but appears much harsher than needed. So isotonic solutions are clearly preferable. Too low salt concentrations let the cells swell up, too high a concentration dries them out and will lead in the mid term to much worse results.

One can use regular salt for that. As long as it's pure NaCl. And one may be 'lucky' in using all other forms of table salt. But regular table salt is not just NaCl!

Nasal irrigation salt needs just to be NaCl used in isotonic concentration. The indian tradition is based on water from rivers, which is not isotonic and now often contaminated.

Clean (i.e. lower salt concentration; but if the water supply isn't safe also meaning: sterilised?), potable/drinking tap-water works as well, but appears much harsher than needed. It is uncomfortable. So isotonic solutions are clearly preferable. Too low salt concentrations let the cells swell up, too high a concentration dries them out and will lead in the mid term to much worse results. The added salt is not needed for the practuce , but beneficial if the concentration is hovering around 0.9%.

One can use regular salt for that. As long as it's pure NaCl. And one may be 'lucky' in using all other forms of table salt. But regular commercially sold table salt is usually not just NaCl!