Element X now has initial support for threads & spaces (as of last week), which were the main things missing from full parity with Element Classic.
Not that many clients have actually adopted it though, because the MSC is still not 100% finalised - it's currently entering the final review stages at last now over at https://github.com/matrix-org/matrix-spec-proposals/pull/418.... Right now Element X uses it (exclusively), and Element Web has experimental support for it; I'm not sure if any others actually use it yet.