Yes, plasmapheresis does remove medications from the patient. In fact, this approach is used to treat intoxication caused by medications (Cheng et al. 2017. PMID 28821193).
However, Cheng and colleagues note:
While [therapeutic plasma exchange] is an effective form of therapy for selected conditions, it does not achieve 100% removal of a target. The mixture of replacement fluid with plasma results in dilution and, ultimately, diminished removal efficiency with each exchange. Typically, one plasma volume exchange removes approximately 63% of plasma contents
So plasmapheresis is not 100% effective at removing the plasma.
Additionally, medications are not perfectly dissolved in the plasma and extracellular fluid. Instead they are bound to proteins and may be sequestered in lipids. Pharmacologists model this effect through volume of distribution, which is the apparent volume that a drug is dissolved in given a particular body weight.
Volume of distribution is different for each drug, a sample list is available from Wikipedia.
The US Department of Health and Human Services notes that an average plasma donation is 800mL.
For 75kg person taking propranolol, the volume of distribution would be approximately 75 kg * 3.5 L/kg = 262.5L. Removing 0.8L at 0.63 efficiency is approximately (0.8 L *0.63)/262.5 L = 0.0019, or less than 0.2% of the total drug amount.
However, some drugs have substantially smaller volumes of distribution. For example, consider warfarin. 75kg * 0.15L/kg = 11.25L. (0.8L *0.63)/11.25L = 0.0448, or approximately 5% of total drug amount.
In short, a typical plasma donation negligibly removes medication from the bloodstream except for those with extraordinarily small volumes of distribution.
Even then, the amount removed is relatively small, and is likely similar in magnitude to other factors that influence drug plasma concentrations like missing a dose.
