I've been reading up on XMPP the last couple of nights and have an XMPP server up and running for some testing, but I'm curious what are some of the more popular public services and what features for privacy do they offer that something like an out-of-the-box ejabberd / openfire / prosody install doesn't do?

It appears that unlike platforms like Signal that encryption from person to person is optional, is there any standard to require it by default with XMPP? Preferably encyption/decryption done on the client side of things so that the server itself could never actually have plain text communication viewed?