Sorry, what's the issue exactly? You called it an issue and I fail to see the problem. The X in XMPP stands for "extensible", so it is being used precisely as intended, so that is still XMPP.

You could use your favorite extensions if you want, but all up-to-date implementations follow the standard defined by XMPP, and it includes all features of a modern messaging experience

If your friends aren't on the same server/client combo then you won't be able to communicate with them (effectively).

You have to be going out of your way to have a non-compliant server or client. This isn't really an issue that happens.

If it were to get a single, matrix-style "spec release" (think an aggregation of existing features into one collection) that contains/requires a bunch of modern chat features I've come to expect from programs

That's how it is today though! I see the issue, you said you haven't tried it in years. Admittedly, I only started with XMPP 2 years ago but haven't had any of the issues you mentioned. Not sure when this became the status quo, but it is pretty awesome. Maybe it is worth trying again :)