When the Pokémon Home was released it was fixed several times, they removed all hacked Pokémon, people asking for impossible Pokémon, people asking for Mythicals etc. Why didn’t they do the same to the Gen 6 and 7 GTS?
It's not a priority. The 3DS is discontinued, Sword and Shield have sold more units than any of the Gens 6-7 games, and Game Freak themselves have more than likely moved on from maintaining the 3DS games. It's likely that GF assumes that the playerbase had moved on as well.