This thread at least implies a basic definition of the Fediverse: Fediverse = ActivityPub.
But there’s more than “uses only ActivityPub and ActivityPub all the time” and “doesn’t understand ActivityPub”.
For example, there is (streams). It can speak ActivityPub. But it is not based on ActivityPub. It’s based on Nomad. It also speaks Zot6. And its ActivityPub support is optional, both server-wide and per channel, although it’s on by default in both places.
And then there’s (streams)’ big ancestor, Hubzilla. It, too, can speak ActivityPub. But it is not based on ActivityPub either. It is based on Zot6. It can speak a whole lot of protocols. ActivityPub support is established by an add-on named PubCrawl which, by default, is activated at server level, but deactivated on newly created channels.
Do both count as parts of the Fediverse because they both have ActivityPub support in some way?
Do both not count as parts of the Fediverse because neither of them is based on ActivityPub?
Do both not count as parts of the Fediverse because ActivityPub is optional at server level? Or because it’s optional at channel level?
Does (streams) count as part of the Fediverse because new channels support ActivityPub by default, but does Hubzilla not count as part of the Fediverse because new channels don’t support ActivityPub by default?
Does (streams) count as part of the Fediverse because ActivityPub support is built into the core, and does Hubzilla not count as part of the Fediverse because ActivityPub support comes from an add-on? (Side-effect, by the way: (streams)’ directory lists actors using ActivityPub, Zot6 and/or Nomad, but Hubzilla’s directory only lists channels using Zot6 in some way.)
Or would you draw the line around individual servers, i.e. Hubzilla hubs and (streams) servers do not count as part of the Fediverse if the admins have turned ActivityPub off server-wide?
Or would you go as far as drawing the line around individual channels, i.e. your (streams) channel officially leaves the Fediverse when you turn ActivityPub off, or your Hubzilla channel won’t be part of the Fediverse until you install PubCrawl?
I think you can look at this in a gatekeeping way and a functional way. I’m personally anti-gatekeeping, so there’s that.
What I mean with functional is: why do you care about whether something is part of the fediverse? Obviously because you want to talk to it. You want to be able to reach it from places like Mastodon.
I see no benefit in worrying whether something is based on ActivityPub or merely supports it as one of several protocols. It remains reachable either way, unless your own platform restricts you like in Lemmy’s case.
That said, I do see your point regarding optionality. I think instances that choose not to use the fediverse plugin of their software aren’t part of the fediverse. But that’s no reason to call the software itself not a part of it. Just means users need to put extra effort into choosing an instance.