@homegrown since Mastodon v4.2.8 it automatically switch new user registrations to require moderator approval whenever they are left open and no activity (including non-moderation actions from apps) from any logged-in user with permission to access moderation reports has been detected in a full week.
And registrations are closed by default on new installations.
https://github.com/mastodon/mastodon/releases/tag/v4.2.8
@ona @pancake @Mastodon