do these still play a role with Compose and min API 21?

Yes, they control the theme of your starting window that the system creates before any code from your app may run

Would there be some future replacement for these?

@Adam Powell makes sense, thank you

@FunkyMuse possibly, but so far we haven't liked the tradeoffs involved in any of the solutions proposed so far, or they've involved longer-term rethinking of aapt and apk resources

Hopefully theres a day-night theme in the future that we can use that doesnt use app-compat so that I can completely drop it.