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I've been prosody for about 10 years now and I've always been happy with it. You can get it at https://prosody.im, and if necessary there's a good tutorial at https://homebrewserver.club/configuring-a-modern-xmpp-server...

EDIT: I might have misunderstood "service" thinking you wanted to self-host.

Dunno. I think you just pick a server you like and use that:

If you don't want to self host I'd recommend https://account.conversations.im/ it's paid but properly maintained to have all extensions for modern experience. If you don't want to pay just pick something with good grades from https://compliance.conversations.im/old/