Following the reveal of a brand new Pokemon, Gimmighoul coming to Pokemon Scarlet/Violet, it appears that its “roaming form” may only be capturable in Pokemon GO in 2023.

The Pokemon Blog states that you’ll be able to connect Pokemon GO to Pokemon Scarlet/Violet in 2023, but you won’t be able to send Pokemon from Pokemon GO into Scarlet & Violet, but linking the two will allow you to catch Gimmighoul. 

This sounds similar to Meltan, which is only capturable in Pokemon GO, and you can activate captures of Meltan once a week by either sending a Pokemon into Sword/Shield or Pokemon Home

I also wouldn’t be surprised if there’s simply a time limit on when you’ll be able to trade Pokemon into Pokemon Scarlet/Violet from Pokemon GO. You can trade them in that direction from GO to Sword/Shield, but you can’t ever trade anything back into GO once you’ve traded it out.