Yes. It's client-dependent for all other systems too. For example Signal's end-to-end encryption only works if both parties are using Signal. For XMPP the diff

Yes. It's client-dependent for all other systems too. For example Signal's end-to-end encryption only works if both parties are using Signal. For XMPP the difference is that there are multiple E2EE clients to choose from (see https://omemo.top/ https://omemo.top/ ). And if someone chooses to use a none E2EE client, you still have the choice whether to communicate with them without E2EE, if that's acceptable to you in the specific context (not all conversations demand E2EE).