I grew up playing games at 15FPS and less, because that's all the computers I had were capable of. And considering I beat some of the hardest games that way it can't be called unplayable by definition.
It really depends on the game.
When I played Half Life (at whatever low framerate my Voodoo1 was capable of) it was a
difficult game. Some of the jumping challenges to get across areas you were trapped in and solving puzzles were a real and serious challenge. I remember saving and repeateldy retrying some areas.
For ****s and giggles I replayed it a few years ago on modern hardware, and with a decent framerate the game was so stupidly easy it wasn't even funny. I played through it start to finish in only a couple of days, whereas back in the late 90's it took me most of a summer as it was a surprisingly difficult challenge.
Where you draw the line between "my hardware makes things unnecessarily difficult" and "unplayable" is really a personal choice though.
It's not that I don't want more FPS, but playing at even <30 FPS is preferable to not playing at all.
I disagree. I would find the experience so horrible and frustrating that I would rather save the game for another time in the future when I have better hardware. In 2023 (and really any time since ~2001 or so) I would sooner not play any game at all*, than have to put up with sub 60fps framerates, at east in areas of the game that matter.
(*with the exception of strategy games, or something else where framerate is less critical)
As an example, there are some areas that are not key to coordination (no battles, etc.) in Starfield that really challenge most modern CPU's, and drop framerates down to the ~55-60fps range. I won't let that stop me playing it, but mostly because these are areas in which your character just casually strolls around, visits shops, etc. If I had to put up with sub 60fps framerates in action or coordination specific scenes, I'd just save the game for later (even if ti is a few years later), and not cause myself story spoilers while having a subpar experience.
Luckily I can manage 4k Ultra in this game in areas where there is action in the 80-110fps range.