The only way to unignore someone on Messenger is to reply to the conversation in the Spam folder. Unfortunately, there’s no way to unignore without replying in any version of Messenger.
Message Requests is also where you’re able to accept or deny messages from people with whom you don’t share a connection.
If you muted a conversation by selecting the Mute option in a chat, you can easily unmute the conversation at any time.
