For Matrix clients I'm given to understand the issue is the lack of XEP equivalents. Either your server(s) and clients are all on the latest available version or you're SOL. This makes third party clients effectively impossible since every change is basically a breaking change and the client and server are tightly coupled.

XMPP took it a step further and has feature segmentation by defining XEPs. This is basic minimum for defining an extensible protocol for client-server communication and has been for decades (maybe it was a new idea when XMPP first started). Notably, browsers use this same thing with w3c specs, which is why they keep working too. If you don't have this feature segmentation and negotiation, you don't have a functional open source client-server protocol full-stop (looking at you Matrix).

I think what XMPP has gotten better at is doing like with w3c and setting baseline minimum features, which has allowed more standardization of clients.