Meltan and Melmetal were introduced as shinies in GO.
Castform's Sunny, Rainy and Snowy forms' shiny variation was first seen in GO.
Also worth a mention that the various Furfrou trims (shiny or not) are only obtainable in Home from GO.
So there is a precedent for some shinies and forms being debuted or exclusively available in GO.
It's a pretty slim precedent, though
Meltan & Melmetal I did forget about when saying that, but also were introduced in (& outside the gift G-Max Melmetan, only available in) Go so there's clearly some bias towards that. I expect Roaming Gimmighoul to get the same treatment (bit surprised they didn't have one to go with the Raid event, actually)
Castform's shiny variations are a bit irrelevant when Castform itself has been able to be shiny since day 1. The fact the forms didn't display as shiny until gen 8 doesn't really matter...In fact, I had a hunch, so I checked the dates.
The first shiny weather form was released March 2021. Castform's shiny weathers were regularly viewable in Home in 2020. The only problem with Castform is it had to wait until BDSP to actually show them.
And Furfrou doesnt matter to this conversation when it was still available before Home. A number of Pokemon were available in Go but not the MSG Gen 8/9 games
It's just not enough to make me think they'll debut Enamorus's shiny in Go before a regular game gets it. Especially since they trend towards dripfeeding everything. Having Shiny Enamorus debut before anywhere else would honestly be a new, non-arguable, interesting precedent to set!
Same goes with other shiny locked Pokemon. Although I could see an exception made for Shiny regular Magearna, since those shiny colors got updated to
actually be shiny look like the anime's "unpainted" Magearna; to that extent we've had shiny Magearna plainly visible for years which imo is fair game for Go. ...After they finally release it in Go normally, anyway.