I wrote quite a bit about MerFight recently: https://www.patreon.com/posts/merfighters-are-157004622. This will, unfortunately, be the last update for awhile due to wanting to redo the netcode, but in the meantime, all stages, costumes, and costume colors are now unlocked, so enjoy!
MerFight 1.9.0 Update
Build and release context
Steam update times within 24 hours of this post. Matching dates do not confirm which build it describes.
Build 23051329 · public branch
Steam updated · First observed