Accounts from offline-instances still show up in follow/follower lists.

[webmind]

This is related to #6054 and #6064, but significantly different.

Expected behaviour

You have people that you follow or follow you.
The instance they have goes down permanently.
The accounts get removed from your follow / follower list.

Actual behaviour

You have people that you follow or follow you.
The instance they have goes down permanently.
The accounts stay in the system and generate a lot of errors on the backend.

Details

Now a difficulty here is detecting whether the instance is down permanently or just is having issues. Some instances have just stopped being maintained, which makes it even harder.
Some indications that could be of use:

• domain name is no longer registered
• server does not respond for X amount of time
• no fediverse instance detected for X amount of time
• SSL is for different host, expired, or otherways broken for X amount of time
• server is known to be offline (the admins have told you so, or some other indication)

X amount of time would be difficult. Another option is to make the purging of this a manual action.

[trwnh]

Purging domains already exists and can be triggered manually, I'm pretty sure?