And here's the commit that changed it: https://github.com/mysql/mysql-server/commit/43a506c0ced0e6e.... It was originally set to max 6 bytes but for some reason someone dropped it to 3.

They also seem to be slowly phasing it out. Internally it's utf8mb3 and utf8 is just an alias. The idea is to eventually make utf8 an alias to utf8mb4.