“Oh, no! There’s a spider in my house! I guess I need to burn everything down.”

All large platforms (except for Twitter) have a trust and safety team to handle those kind of situations. They also establish relationships with the various government agencies and non-profits that deal with that kind of stuff.

If they’re replacing chat with a new system that isn’t compatible with the old that means either migrating and modifying all messages to be made compatible -or- keeping part of the old system online while adding adapter code to the new system so old messages can be fetched and displayed. Reddit is going the cheap, easy, and anti-user path of deleting old messages.