Actually, they somewhat recently added phone numberless registration. It does require a $2.99 fee however, apparently to prevent abuse.

Or XMPP, if configured correctly. There are others as well.

Yeah fuck phone number based services! Tho i would like to mention, matrix isnt the only service thats private, take a look at reticulum for example.

What do you think of SimplexChat? It does not require a phone number or account setup and you share single use links to set up a chat so there's no identifiers that can be traced back to you. I've been using it with my friends for years now. It has some reliability issues but you can even host your own server if you want full control.

Ive been using Disroot conversations. No phone number required