If you want to unrestrict someone on Messenger, tap on the Menu icon (three lines) on the top-left of the app, then the Settings gear icon to enter Settings. Next, tap "Privacy & safety," then "Restricted accounts," and then tap on the person whom you want to unrestrict. You will then be taken to your chat with them, and there is an option to tap "Unrestrict" at the bottom of the screen to remove the restriction.