Spam has consistently been the death of the open internet, even the big tech silos struggle with spam (Instagram for example – despite having incredibly invasive techniques for identifying “genuine” users – is STILL inundated with spam commenters). I think instances on the fediverse should reconsider their open registration policy, either totally close registrations when you reach an agreed upon critical mass of users, or adopt some form of invitation or application system for new users. I believe Mastodon supports both in the software.
How have you determined these communities are no longer a legal concern?