Signal is asking phone number to use, I don't see how this is good for sensitive communication (since metadata alone are often very informative).

Exactly. I have found the phone number requirement idiotic from day one. I recognize the decision was made as a trade off between usability and security (enabling discovering friends via phone etc), but they seem unwilling to admit that this does compromise security.

I've thought about a phone-numberless messaging app, but then it'd be full of spammers.

Is that really a problem though? Most people would just add their friends via username and bypass the whole user discovery process. Discord has demonstrated that this works perfectly fine, even with anonymous accounts not tied to emails.

I’ve only used discord a couple months in a handful of servers and I get spam dms..

that problem was solved like 25 years ago, just ignore everyone who you didn't seek out yourself by default. Basically, make liberate use of the block functionality.

Matrix seems to be doing just fine.

Any messaging app that allows users to communicate without E2EE is actively harmful.