These days you really better pay attention what you're buying and what kind of ecosystem you're buying into.
I get why they check if it's children with accounts they're not supposed to have... I once saw a documentary about VR. And there are lots of adults enjoying adult content. Mingling in virtual bars and clubs and doing adult stuff. I'm not sure if VRChat etc are available on the Meta devices... But it's not great that children are in those virtual areas. Not for them and not for the other people who want to do their thing. So I get why they're cracking down on this and forcing people to use the correct account.
However, requiring phone numbers, ID and credit cards is ridiculous. And lots of services do it. Google also restricted my account (for claimed suspicious activity) and now they want my ID. And I refuse to provide it.
Hmmh. Why ActivityPub? I mean I suppose it's alright as a standard for some turn based or slow trading game. But it's neither very efficient nor suited for realtime. And having long (and descriptive) JSON messages, queues, ... is baked in per design.
And it's not even interesting to a Mastodon user if player x sold y latinum to player z. So for lots of game logic we don't need messages in a common format that's federated to Mastodon, Lemmy, Peertube etc.
I think a nice and not too complicated coding challenge would be to design a world that spans multiple servers. Players could roam a world, go through some door or portal and the client seamlessly connects to the next server. So that part of the world (the other server instance) is behind that portal. That'd make sense from an in-game perspective and won't be that hard to implement. Basically it's just like any other game, just that the client auto-connects to servers with some internal logic and not just in the start menu. And ideally authentication would be federated. The new server could ask the player's home instance to authenticate them on entering the new instance.