I am very skittish around iOS ecosystem, and am responsible for keeping a 6+ year old iOS app running. It's not a very complicated app and it uses almost no permissions or services aside from camera & GPS, but we got burned a few times with breaking iOS changes over the years. I think we started w/ iOS 8. That said, I haven't seen a breaking change since 11, and I just got done with some initial test scenarios on 14 without concerns.
I just told my team to go ahead and update to 14 because at the least our app will open and run through some basic scenarios without any issues. Now I just need everyone to exercise all the code paths over the next week to be sure.
To be very clear, our app is not on the public store and is distributed using the enterprise program. This allows us some freedom as the Apple devices running our app are exclusively for the app and we have some leeway in telling our customers to hold off on the latest iOS if we need to. They have MDM and policy which allows for this level of control.
If we had this app on the public store, I would be in a state of total panic right now.