(And in an episode of Pulling Stuff from my Archives...)
I'm fairly certain that Android versions 5 and later will be supported by apps for increasingly-long periods of time as the last version stops being supported, almost turning them into Windows XPs; characterized by impressively-long de facto support lifetimes. I think most could get away with 5, and I think most everything would run on 7. The latest version is 11.
There's just not that much reason for people to buy phones frequently anymore. Mobile hardware and software are maturing, and it's no longer this massive series of growth spurts like computers were back in the late 90s and early 2000s. Sure, manufacturers can try to artificially stop support after 3 years/version updates, but it's not like any app worth its salt would only run on the latest version anyway. Market factors dictate that they have to go as far back as 5 or even 4.4.4 to broaden their audience as much as possible. Performance isn't that much of a concern anymore, either, since you don't really need a Snapdragon 875+ to watch Youtube or browse the web.
I'm not saying "buy an old-ass phone" if you can buy a new one. Just buy a low-end or mid-range new phone if that's all you need. Their processors are made on newer, smaller manufacturing processes, and they're more power-efficient because of that. Plus, you get at least 2 years of official support minimum, so that's nice, given that those include security updates.
I know that Apple's very similar; their iPhones are officially supported for, like, 6 years, IIRC. Only difference is that Apple support is official, while Android is de facto supported by apps for about that long or longer.