I’d be ticked if I found a Shiny like that and wasn’t allowed to keep it. Not like it matters with the transferring restrictions anyway. That feels like a dumb thing to be told “Hey, you’re not allowed to tell people about this”- maybe they’re trying to leave it up to suspense or something, “buy the game and you’ll see!” kind of thing, but, like… no? If I were in charge I would just come out and say whether or not the sparkling effect is back. This is such a stupid thing for them to try and keep a secret.
The exact statement Serebii’s site is “Alas, I was unable to find a Shiny Pokémon in it but have a third-hand account of it. Unfortunately I can’t confirm first hand if there are sparkles or sound but word is that we may be very happy.”
That reads to me more as him being unable to say due to a lack of verified information (and thus not wanting to make a claim in full confidence) rather than because of TPC mandate.