i dont think it is the thermistor degrading. To answer your question directly, all my thermistors have failed because the tiny wire has broken off, and generally the fail is “hard” not soft.
It is more likely you’re experiencing absorbed humidity in the filament from it being out on the spool too long. this makes you have to increase the temperature.
I’m sure if you dehydrated the spool in the oven for a bit you’d get better prints and a slightly lower print temp.
Additionally, keep in mind that a lot of PLA on the market nowadays runs at higher temps because it’s laced with something (“PLA+”?). Check the item description for indicators of a new or improved formula.
You could try ordering a thermistor of the same make and testing the temperature of it. You can get a half dozen for about $10. The Marlin firmware your printer probably uses should have a section for thermistors and what kind you’re using. It’s probably type “1” which is the cheap kind.
You can try using one of those distant head thermometers to measure the temperature wirelessly at the hot end. These are probably over $100 right now because of COVID. I think that is how a well-resourced person would handle this (i'm not one personally).
Maybe start with the oven and while it’s cooking, research the thermistors and what goes in to replacing one. It’s a good learning experience because one day your hot end may be irreparably clogged and you’ll want to know some of the steps to disassemble it.