that's hard to quantify as games with very low framerate variation can imo still look pretty fluid and have little impact on playability at 25fps, while games that always suddenly drop to ~27fps and then go back to 30 and so on can feel very choppy and annoying

Framerate is not really an issue unless it is in a high movement game. Stability of framerate is where you get problems of people noticing it. You may not notice 20fps consistent rate as bad, but a game that jumps between 24 and 60 and everything in between will be annoying as hell.

I can play at ~25 but it'll bother me, just not enough to stop playing. No less than that
Ideally it should pull at least 30 with minimal drops (like when explosions with a lot of effects happen)

30 is too low, especially now that I've grown accustomed to 60+ in all my games. The Witcher 3 dropped to 45-50 (70+ without) with Nvidia Hairworks on and it bothered me like hell, games with high draw distance will display greater disparity in frame rates overall.

Unacceptable would be ~20FPS, okayish would be 25 FPS.

lower than 20.

I was playing Quantum Break on my rig...it's so difficult to get the game smooth always at 60 fps 1080p and most games until now. So for me anything as less as 25 fps will be sort of game breaking.

depends on the game

Unless it's 5th gen, I don't want to see anything less than 30. I can deal with occassional dips to high 20s I guess.

15-20 Already is too low because is hard to play this way, and 30Fps isn't too low, back then the games don't had more FPS than that, "Too Low" means Unplayable, and 30Fps, of course isn't Unplayable.