Game has a database with items/thralls/everything else in it probably.
They did a rollback so they had a backup and therefore the state of that db before the incident.
You have the state of the db after the incident and the event logs.
Just check for those people who got decayed thralls in that day before the server setting was changed. Then restore the thralls from the backup’s database.

Why people say “oh rollback only solution! Only!”? It’s because they don’t know or they are accustomed to this low-effort service FC provides.