The game is right around the corner so I call dibs on this question.
No guesses, however datamines and confirmed sources are a-ok
If there’s no confirmed answer after about a month, a solid “it’s unknown BUT” would suffice
Anyways have a good day
Right now, the best and only shiny hunting method involves Outbreaks. Depending on your research level of the Pokemon in the outbreak or if you have the Shiny Charm or not, you can get odds from 1/158 to 1/141. Right now it is not known the optimal way to shiny hunt these Pokemon.