Adam Lear’s answer is correctAdam Lear’s answer is correct, but there is one other possible (and in a way, more innocent) reason for extremely long suspensions: The Super User moderators at some point started to merge all spam accounts into a single user (this has several interesting implications for spamfighting) instead of outright deleting them. The target user, into which 487 users have been merged to this date, is currently suspended until 2025. There’s obviously no reason to ever have this “user” not be suspended.
I don't know if other site moderators have started doing this as well.