Lastly, keep in mind that sso-oauth is not deprecated, and will still be maintained. It's still a reference implementation for OAuth2 providers that follow the standard loosely. Also it happens to be the only OAuth 1 reference implementation.
I've said it there and I'll say it here. NodeBB will follow the ActivityPub spec and, if possible, will be tested against both Flarum and Discourse integrations because doing so makes federated forums a force to be reckoned with.
This is good. More and more development efforts too are focusing so little on the Mastodon API and instead focusing on the actual standards - S2S & C2S as they should.
Building Federation for the Fediverse is of paramount consideration, while making sure one federates with a single Fediverse platform whose significance is waning as the bellweather for social communications. Even entire Mastodon communities have left that platform and migrated en masse to other, more capable platforms - some outright forks like Glitch-Soc or Hometown, and many more to other more promising Fediverse platforms that have enjoyed greater adoption due to their more accommodating utility for their userbases.
In the End, it's not really about communities built on an instance and provided to the user base - it's about users across the entire #Fediverse building communities that span the myriad platforms and instances; especially as smolweb / smallweb and single user platforms continue to build critical mass in terms of installed instances.
#Friendica, being one of the great-grandaddy's of, and staples in, the Fediverse, is a great platform chosen by many projects to effectively represent the branding, marketing, and social communications needs - blogs, announcements, changelogs, newsletters, and public dialog... Things a limited microblogging software cannot deliver.