Alexander Bochmann is a user on mastodon.infra.de. You can follow them or interact with them if you have an account anywhere in the fediverse.
Ok, I give up after going through my following list for 45 minutes, trying to clean up nonexisting and migrated accounts...
Using the list in the Mastodon pane UI is nearly impossible (jumps back to the top after selecting an account). When logged in, you can use the /following page on the public web UI with additional functions - there's an "unfollow" button on there.
The button is missing for migrated accounts though (why?), and anyway there is no indicator on account reachability.
@galaxis had the same experience, so I was also giving up after a while... with that UI I can not clean up my followers list...
So, that means opening the web page for each account in a new tab.
Some still exist, but have been unused for various amounts of time. Some have expired ssl certificates. Some have broken installs. Some have something else on there now. Some are completely offline.
Which of the problems are transient? Which have existed for a long time? No way to know.