My guess is that it’s beyond end of life.
Spinning up a new vm and switching to it requires zero downtime and if something goes wrong you just stay on the existing server. It’s a bit complicated though.
Sorry that you had no choice but to go to an old backup. I’d switch to daily backups and take a backup before you update so you won’t need to lose anything.
You can switch to the beta or stable branches before you upgrsde. You can read about Configure a supported tracking branch to get Discourse software updates