I agree with you that, "Old and functional does not mean deprecated." but what you asserted is not what deprecation is either. It generally means the developers are giving notice that new development on a feature/feature-set/api is discouraged. That notice was given for X a while ago.

I understand the idea that it would be better if the replacement were 100% ready when that notice was given, but I'm not surprised that wasn't the case with X, it's big shoes to fill.

I can't speak for normal users, but I have seen some reporting, that they have been on a Wayland for a while now without knowing it.