Currently compose needs minSdk 21. Will the final released version support lower API levels?

we're not planning on it, no. api 21 devices have been out for a solid 5 years at this point 🙂

More specifically API 21 guarantees a few important things for us, such as render nodes, ART (better GC), render thread, etc.

Not to mention the overwhelming majority of users are on 21+

TLSv1.2 !

Well unfortunataly a solid 10% of users are on lower API levels. I hope the situation will change by your release time so that big/medium commercial apps will be able to move over

At ADS Romain said that Compose should be in beta in 2020. So I guess it’ll reach 1.0 stable in a year or so and at that point Lollipop will be 6 years old.