So one of my contacts is now in this position, phone lost. We chucked their lost account out of group chats, but the chat history and contacts are in someone’s hands. The device is PIN-protected but they hadn’t even enabled the numberpad shuffle, let alone an a password/phrase, so that probably won’t help.
Everyone who was in contact with them could rotate accounts. Each person could create an account, add it to all their groups, give manual warning, then remove their old account from those groups. It has not yet happened.
I don’t think there’s anything we can do about the history.