https://matrix.org/docs/spec/#matrix-versions. Matrix 1.0 is the blanket term for the set of various specific API releases we just cut: CS API 0.5 etc. We could have bumped them all to 1.0 for the sake of it (and perhaps we should have), but it felt cleaner to let each API version evolve independently, and instead say “hey, as of this set of versions, we consider the protocol stable”.
https://matrix.org/docs/spec/