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.
The Spam folder has been deprecated. Unfortunately, you won't be able to recover those messages.
I have just one thing to say if that can help you out. Please visit Facebook and go to the activity log, if you remember the date or month filter it in the search and there you may be able to do some action. I don't know exactly but I think you may find a way there.
Let me know further.