I have logged out during the Lich King era (before you could actually fight him) and I haven’t logged back in since, but I last week I decided to try WoW again after 10+ years of absence.
I bought the epic Shadowlands expansion and I now have a thirty day subscription. I can login fine on all of my new characters on Neptulon, but when I try to login on any of my old characters then I keep getting the ‘World server down’ message.
I logged in on an old level 10 character just fine before the subscription started (he was standing in Ironforge), but now I can’t login on him anymore either. The loading screen goes to 80% on all of those characters, then it’s stuck for a while and after that the message pops up.
I could rename all the characters without issue, but I can’t seem to login on them.
I’ve tried moving my old character with the automated moving system, but that doesn’t give me any results either. It says that the character will be transported to Westfall (I’m playing Alliance), but on the character screen it keeps saying that they’re in Ironforge.
I did manage to login on an old Death Knight that was in Hellfire Peninsula, but none of my other characters can login.
I can login on my newly created characters without issue. I’m playing on Neptulon (low population server). I have this issue with old characters on other servers too.
I’ve also tried disabling and removing add ons, but I do not have the impression that this is the issue. The Death Knight character could login fine with add ons enabled while removing them and renaming the WTF/ Cache/ Interface folders and restarting WoW made no difference.
Can a GM please help me out and fix whatever is wrong with my old characters? I have the impression that they’re stuck in limbo from all the updates that occured in the past decade. The automated systems aren’t helpful and I would like to play my old characters again.
A small update: I have done some more testing, and it’s only the characters that are currently in Ironforge or Stormwind that appear to be affected.
Unfortunately, this happens to be all of my high level characters and the bank characters I had that undoubtedly stored many riches back in the day. 
(Whether those riches are now extremely rare or completely useless remains to be seen, but I’d like to find out regardless.)
Update 2: it seems like whatever is causing this issue spread to other characters and I can’t seem to login on new characters either now. I figured I’d play on a character that I created Friday, but alas, I receive the ‘World is down’ error there too now, even though this character is on Argent Dawn.