It’s the issues with XMPP’s spec: you don’t just use XMPP, you use XMPP + your favorite optional spec implementations.

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.

You could use your favorite extensions if you want, or follow the standard XEPs that all up to date client and server implementations support.

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 :)