I'm not sure where you got this information. You can't transfer a Pokemon to Sword and Shield if they're holding an item.
Items can’t be transferred to Pokémon Home or Bank, period. Mega Evolutions don’t exist in Sword/Shield.
Mega stones are in the coding and I've seen people spawn them in when the hacked raids were a thing but they have no use
This means that you can attach a Mega Stone to a Pokemon if you hack one in, but nothing will happen. Only Game Freak knows why they didn't remove them. It may have simply been easier to keep them in, rather than taking them out.
Hope I helped!