It seems Meta recently migrated all Messenger chats to e2ee. But if you had created a secret conversation (e2ee) in the past then instead of migrating the existing chat Meta decided to archive and switch to the secret chat.

Since I only experimented with/created secret chats in the past with close friends and family, all my old conversations with years of history were archived with no path to restore.

This is a horrible UX experience and absolutely ridiculous that this design passed in Meta.