There are a couple I have in mind. Like many techies, I am a huge fan of RSS for content distribution and XMPP for federated communication.
The really niche one I like is S-expressions as a data format and configuration in place of json, yaml, toml, etc.
I am a big fan of Plaintext formats, although I wish markdown had a few more features like tables.
Those problems you speak of about XMPP are not really a concern anymore and haven’t been for a while.
Matrix on the other hand is very difficult to implement, and currently there’s only one (maybe two?) viable implementation choices. It is way over complicated, resource intensive, and has privacy issues.
Does it have privacy issues compared to XMPP which doesn’t enforce the privacy extensions? I figure they are about the same there. Asking genuinely as I do not know other than Matrix might leak some metadata.
And quite frankly, I really wish we’d just agree on one or the other. Would love to host an instance and move some people to it but both are just stuck in this quasi-half used/half not state. And even people on here can’t agree what should be “standard.”
Xmpp definitely wins in privacy. What is there to privacy more than message content and metadata? Matrix definitely fails the second one, and is E2E still an issue for public groups? I don’t remember if they fixed that.
XMPP being a protocol built for extensibility means it will be hard for it not to keep up with times.
On your point of picking one or the other, I’d say pick the one you like and bridges will help you connect to the other. But XMPP came way before matrix, and I believe they fractured the community instead of building it.
There’s a good reason all the big techs built on top of xmpp (meta, Google, etc). It’s a very good protocol and satisfies modern demands very well.