I had a really old phone that I liked, but which slack discontinued support for. The effect was that the slack app would still start and you could even see the channels etc behind the big "not supported" badge that immediately popped up. However, notifications still worked, and that was (and is) my main use case anyway, so I didn't mind. I found it a pretty weird way of "discontinuing" support though.
Unfortunately I had to switch to a newer phone later when my bank app started complaining.
It makes me really sad that some modern apps don't work on older Android versions. For the apps I'm currently working on, I'm making a big deal out of them being responsive and usable on 8 yr old phones.
Unfortunately I had to switch to a newer phone later when my bank app started complaining.