You should try it again instead of spreading very outdated info about it. All major clients and server implementations have more or less feature parity and interoperate fine these days (and yes there are yearly complicance suites for XMPP that are exactly what you are asking for). What you are saying was true 10+ years ago when Matrix didn't even exist yet, and Matrix has very similar issues with different client and server combinations these days.

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, 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 :)