This is an interesting point for me, as my iPad 8 is on the OS it shipped with (14) and I will never update it. Not a fan of the way 15 and 16 have gone on my friend's 8. Some of my apps are already out of support (irrelevant for now), but the primary third party app I use is YouTube which is still supported, even once that falls I can use the web version.
I do wonder though when the web browser will become unusable. It's encouraging to hear that 12 is still working for the most part. iOS 14 is already 3 year old software, and that's my observed usable timeframe for desktop web browsers. For example, the newest Reddit website design is completely broken on 2019 Firefox versions, but works on 2020. I've studied this a bit for a few years as a user of old Macs.