Ie similar to how Lemmy allows users to create instances that are federated with each other, only this Discord alternative’s instances wouldn’t federate with each other.
Instead, each instance would be a complete silo, although instance owners would be given the option to potentially federate their instance outwards (ie to other federated platforms), if they wish.
Perhaps it could potentially be forked from the already existing Lemmy?
https://www.windowscentral.com/software-apps/discord-alternative-search-10000-percent-stoat
https://www.zdnet.com/article/discord-age-verification-requirement/
XMPP and Deltachat are federated (though not with each other). A person can self-host an XMPP or Deltachat server that can communicate with any other XMPP or Deltachat instance.