The PubChem article you linked (National Center for Biotechnology Information, n.d.) stated that
When heated to decomposition this compound emits very toxic fumes of SOx and NOx. It may also emit toxic fumes of carbon and COx. (NTP, 1992)
So what is the required heat for decomposition?
Saad, et al. (2014) points out that
Saccharin decomposes at 228°C in acid (Lide, 1997); > 300°C in sodium and calcium salts (Mitchell & Pearson, 1991)
Cooking and baking does not often involve these kinds of temperatures so therefore this fact is irrelevant to food safety.
For ordinary cooking, the maximum temperature used is 287.7 degrees Celsius (Nathan, 2005), so it is only when cooking at the very hottest of temperatures that you need to worry about this fact.
As for the carcinogenicity, of Saccharin they also state that
Arnold (1983) studied two generation saccharin bioassays. Results showed that humeral antibody production in rats is seriously affected that may lead toward cancer. In 1977 FDA proposed a ban on saccharin use due to cancer reports in laboratory rats (Arnold, 1984; Tisdel et al., 1974; Schoenig et al., 1985; Taylor et al., 1980).
The results of the study by Arnold (1983) resulted in the prohibition of saccharin in Canada and a proposed ban in the United States. This proposed U.S. ban was withdrawn in 1991, but foods containing saccharin were required to carry a warning label. This warning label was placed on all products containing saccharin to indicate “saccharin is a potential cancer causing agent.”
Supporting evidence for carcinogenicity given by Saad, et al. (2014) includes
Exposure to pure saccharin supported its role in pathogenesis of the liver damage (Negro et al., 1994). Several studies have been done which shows association between bladder cancer and saccharine (Fukushima et al., 1986; Shibata et al., 1989; Cohen et al., 1991; Ito et al., 1983; Fukushima et al., 1983; Fukushima et al., 1986). All of the ingested saccharin after circulation in blood excreted through urine from body (Sweatman et al., 1981).
However, Whitehouse, et al. (2008) points out that (emphasis mine)
A 2007 article by Gallus et al. discussed artificial sweeteners and associated cancer risks. The authors reviewed several case-control studies and found a lack of association between saccharine, aspartame, and other sweeteners and several common neoplasms. They did cite an ecology study that indicated a direct correlation between brain cancer and aspartame consumption. These studies are subject to ecological fallacy and the hypothesis was not confirmed in animal or human studies.
Research since Arnold (1983) showing the safety of saccharin led to the overturning of the decision to warning label products containing saccharin in the US in 2000. A ban on saccharin still exists in Canada; however, Health Canada is currently considering relisting saccharin as a food additive (Whitehouse, et al. 2008)
Arnold, D. L. (1983). Two-generation saccharin bioassays. Environmental health perspectives, 50, 27-36. doi: 10.1289/ehp.835027
Gallus, S., Scotti, L., Negri, E., Talamini, R., Franceschi, S., Montella, M., et al. (2007). Artificial sweeteners and cancer risk in a network of case-control studies. Annals of Oncology, 18(1), 40-44. doi: 10.1093/annonc/mdl346 Free PDF retrieved from https://academic.oup.com/annonc/article-pdf/18/1/40/794927/mdl346.pdf
National Center for Biotechnology Information. (n.d.) PubChem Database. Saccharin sodium, CID=656582. Retrieved from: https://pubchem.ncbi.nlm.nih.gov/compound/Saccharin-sodium
Saad, A., Khan, F. A., Hayee, A., & Nazir, M. S. (2014). A review on potential toxicity of artificial sweetners vs safety of Stevia: a natural bio-sweetner. Journal of Biology, Agriculture and Healthcare, 4(15), 1-12. Free PDF retrieved from: https://pdfs.semanticscholar.org/fc15/178ca15502d3c0d64e7753eccb88e7bd73db.pdf
Whitehouse, C. R., Boullata, J., & McCauley, L. A. (2008). The potential toxicity of artificial sweeteners. Aaohn Journal, 56(6), 251-261. doi: 10.3928/08910162-20080601-02 Free PDF retrieved from: https://www.researchgate.net/publication/5247958_The_Potential_Toxicity_of_Artificial_Sweeteners
Nathan, M. (2005). Temperature of a Home Oven. The physics Factbook Retrieved from: https://hypertextbook.com/facts/2005/JenniferZhong.shtml