It was most likely taken down because it is one of those mods that replaces one game with another game that SEGA doesn't have the license for. (i.e. Replacing Virtua Fighter 2 with Mortal Kombat, a series currently owned by Warner Bros and NetherRealm Studios (It used to be owned by Midway Games, but then the company went bankrupt and had to sell off their assets.))

Every now and then the Workshop gets purged of those kinds of mods, because if these kinds of mods are left there, it may create some legal trouble for SEGA.