Lichess had an upside down chess tournament. It was very much fun to watch. Several GMs joined and were streaming. GM Anish Giri streaming upside down chess : https://www.youtube.com/watch?v=0Jxu_9eaUz4
FICS (freechess) had this variant as Wild/5 for years but it was not very popular. White has a clear opening advantage. If you manage to bring a white knight to d6 without it having captured it could be checkmate already. Because everything is upside down one can promote a lot of queens.
Forum discussion : https://lichess.org/forum/general-chess-discussion/upside-down