Secret Club Necrozma is able to uploaded to Pokemon Home, but not usable in any online content inside of Sword and Shield..
Naturally it's unable to be traded on the GTS due to it being a "special pokemon".
I came to ask if anyone has found a dumb loophole like the Solgaleo/Lunala issue at the moment.
I tried making an illegal Necrozma (with the same situation as a tradeable Lunala/Solgaleo), but it gives the same "problem" message as if it was the event one.