sakurajima.moe is one of the many independent Mastodon servers you can use to participate in the fediverse.
A comfy, non-toxic, and inclusive Anime/Manga/Japanese Media fandom and Japanese Media/Culture Content Creators (Bloggers/Artists/Video Creators/Podcasters/etc.) Mastodon Server

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@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.

misskey-hub.net/en/docs/releas

Misskey HubRelease Notes | Misskey HubRelease Notes for Misskey.