I'm talking about Community days, Outbreaks, Festivals, Adventure Week, etc. I have only found shinies through events. (Most recent shiny Plusle). However there was that time I caught shiny Charmander 3 hours after the upped spawn rate.
You can find shinies outside of events. The chance of finding them is just lower since the spawn rate of Pokemon that are shiny are lower.
I've gotten a shiny Bulbasaur outside of community day, and have gotten Swablu - which was never part of an event.