Bulbapedia didn’t say anything about the Shiny Chances on these forced encounters.
These encounters aren’t listed on this site’s list of Pokémon that cannot be shiny.
However, on this other site, it says:
> As we’ve said, pretty much any addition in Sword and Shield can be Shiny apart from a few notable exceptions. We’ve listed them out below for your reference so that there’s no ambiguity:
-Starter Pokemon
-Zacian, Zamazenta, Eternatus
-NPC trades
-Wild Area Pokemon that can’t be caught
The forced encounters can’t be caught since you will not have Pokéballs by that case, but I believe that’s reading between the lines. Also they’re in the Slubering Wealds, not the Wild Area.By what I see it seems the forced encounters aren’t shiny locked. Hope this helped!