> ...but only if all the servers support the right extensions...

That is only for OMEMO (OpenPGP and OTR require nothing of the server) and you can easily check a potential server for the things that OMEMO depends on by doing a normal server compliance check here:

* https://compliance.conversations.im/

In the same way, if you pick a client that does not support something then that thing will not work. But why pick such a client in the first place?