Your question is kind of confusing, KDE does not do anything specifically with Manjaro, I assume that Manjaro users have to wait for upstream (Arch BTW) to mark it as stable, so if you keep your eyes peeled for Arch releases of Plasma, I'm sure you'll have an idea of a timeline.
Tbf, Plasma 6.0.0 was not exactly bug free. Then again, Plasma 6.0.1 was the first release on Arch and for me, it didn't break anything. Only some UI issues and occasional (although never fatal) crashes.