@helge This stuff can be done client-side.
That's why #Pachli allows users to block / hide-behind-a-warning notifications and DMs from accounts you don't follow, or are too new (https://pachli.app/pachli/2025/02/28/2.10.0-release.html#anti-harassment-controls-for-conversations-private-mentions).
An API change (https://github.com/mastodon/mastodon/issues/33066) would make this easier, but I think people fall into the trap of thinking this all has to be done server-side too often.