According to an ex-committee member I know apparently the site was attacked and destroyed by ransomware hackers, and the backup failed as the hosts screwed up.
The last I heard the SSVC are working to retrieve as much as possible from archives, but that was March 2021.
Had a reply from the SSVC President yesterday.
"Hi Simon,
As you're aware, we lost the forum last April to ransomware and have moved to a completely new website and membership system. There is no forum in place at the moment, but we hope to have one soon. Unfortunately none of the previous user names or passwords are valid.
Thanks
Dougie"