I like the idea of using emoji to communicate to the team info on a given PR. I have seen similar practices and emojis used by different teams I have been a part of. I really like it since it helps saving time to the author, the reviewer and other team members that know if someone is already looking at a PR or not.
Also, a favorite of mine is to have a merged emoji to indicate that the PR was merged. I also like to give in slack in a thread a little ack and thank you to the reviewer.