What? This all started with iOS 7. Bugs introduced in 7 have STILL not been fixed. I lucked out and skipped it, but then I learned from fellow users on forums like this that the bugs I was reporting to Apple in iOS 9 were introduced in iOS 7. Out of the 50+ bugs I've reported, only one or two have been partially or completely addressed... in over SIX major releases.
Standard YMMV disclaimers apply. I would agree the downward slide started before iOS 9 (of course it did) - you certainly can have more insight depending on your exposure to other areas of the OS. But IMHO 9 is where the entire OS went off the cliff.
For example, iOS 9 was BRICKING phones due to Game Center bugs when it was released.
I'm a dev and released a game as iOS 9 was being released. We had Game Center fully tested on iOS 8 and ready to go. But when we tested on the iOS 9 GM, EVERYTHING broke. We spent almost a month developing workarounds for most of the bugs we found in the GM, but could not implement workarounds for the Game Center disaster, so we had to pull all Game Center features from the Game.
We learned valuable lessons from the experience:
1. Forget about being featured or any "Curation" if your app does not "worship" all the latest Apple OS features. Releasing without Game Center was an unforgivable sin. Think of Apple Platforms as "alters" that you worship at. The life of your business cometh from Apple - no, you didn't build anything.
2. If an Apple OS platform feature is SO BAD that you must remove app features that use it (like us pulling Game Center support), then you must delay releasing your app until Apple fixes their bugs. Of course you have no input to when or if Apple will fix their bugs, but that is your problem. Submitting bug repots to Apple is a useless sham - using your VC connections is the only way to get Apple to fix their bugs. If you don't have those connections, AND enough VC cash to keep the lights on while you wait for Apple, then you don't really matter anyway. So do whatever the F you want, but Apple prefers you just "Piss Off".