(I catch a Shiny on camera during the tutorial and get excited haha)
I have 9 Shiny Pokémon after 65 hours, most of which I was guessing at these methods before datamined numbers were known.
Partener starters are shiny locked, but later ones are not.
Gift Pokémon are shiny locked.
Legendaries and Mythicals are locked.
Cave Alphas are Locked.
Non outbreak shinys – You can save when they appear and they stay after a reload. You can teleport away within a zone, and even black out, and they remain, as long as you don’t head back to Jubilife.
Shinys are visible / audible at full flying height from ground. If you launch off the ground, you won’t be too high to find Shiny Pokémon.