This same thing also happened with tetrisfriends.com. It sucks because they took all of the game modes that you could play for free and then are willing to sell them back to you in Tetris 99.

Also I don't really follow any Tetris forums or anything so I don't know how people reacted when it shut down but I was amazed it shut down since they sold mtx skins and I assume that they didn't transfer any of it to Tetris 99.