I would encourage every Fediverse software project to implement a “dead-man switch" on registrations: if nobody with moderator permissions has been active in the last week, then disable new account creation.
The Fediverse has a significant number of abandoned instances that are used by bad actors to create accounts and send spam.
We implemented this in Mastodon (https://github.com/mastodon/mastodon/pull/29318) and it has been highly effective.
@renchap It seems that Misskey have implemented this feature in the recent release of Misskey (2024.10.1)
This is translated from their release notes
As an anti-spam measure, moderators are now automatically switched to invite-only (Control Panel -> Moderation >-> “Allow anyone to register” turned off) if they have not seen any activity for more than 7 days. ( #13437 )
Moderators will be notified when the switch is made. If you want to keep registration open, please set it up again in the control panel.