[goffi]

XMPP does use Atom as its (micro)blogging format (XEP-0277), and making followers/following lists (subscribers/subscribed in XMPP terms) public is opt-in with XEP-0465.

Note: I'm very involved in XMPP, and the author of the latter XEP.

Edit: forgot to mention that it's also available to ActivityPub thanks to the XMPP <=> AP gateway (that I've authored too)