Encounter:
Golbat in S/M, while randomly traveling through Mt. Lanakila. I don't have Shiny Charm yet at that time, so it was a full odds shiny encounter.
Meanwhile in US/UM, I already got the Shiny Charm before I even reached Mallow's trial, and yet no shiny encounter until I reached postgame lol. My first shiny encounter was Blacephalon during postgame, which I intentionally hunt.
Non-encounter:
None in S/M, since they shiny locked literally everything that's not the starter, and the starter scene is so long I couldn't be bothered to SR.
In US/UM however, I successfully SR-ed for a Shiny Rowlet as my starter.