I marked a conversation as spam by accident (wanting to press mark unread) from Facebook desktop messenger and it got removed without ANY warning from my conversation list. Any way to revert this mess? I tried checking spam messages in mobile Facebook and see filtered requests but no luck.
Go to the app-version of messenger. Click the chat head in the left upper corner with your own image. A menu appears. Choose message requests. You come to a folder separated in two categories. Choose Spam. These are the messages marked as spam either by you or Facebook. To get it back to your inbox, the only solution I've found so far is to reply to it, but I'm trying to find other options.
The Spam folder has been deprecated. Unfortunately, you won't be able to recover those messages.