There are different degrees of vendor lock in. If you use email (or Matrix) with a domain, you have no control over, you are soft-locked it. You can buy a domain, self-host or pay for a managed service and inform everyone that you are now reachable over some other address, but nobody else has to change.

If you use Signal (or Discord or whatever) and want to switch to a different domain. You cannot. If you switch to a different protocol, everyone in your contacts has to switch as well, or you lose that contact. The network effect forces you into the service of one provider. The only way out of there would be if the service get so bad, that a critical mass leaves, but you will have to deal with that bad service all the way.

As long as financial interest are there, non-federated services will sooner or later start to enshittyfy. So if you choose a communication medium, choose something that leaves your options open. If you don’t like Matrix, try XMPP, it has come a long way as well.