With many apps an issue for changelog writers is that often behavior is tied to feature flags, which can be remotely enabled. Thus a new feature is rolled out in the app but only visible to some testers. Only after a while it's enabled for all users. Thus app release is independent from availability of a feature.
That said "Bugfixes and performance improvements" is as useless as it gets ...
It depends on the feature. For things that involve material changes to your app or a feature that you don't want Apple to be surprised about, usually you will include user credentials in your submission that have that feature enabled and give them a heads up. If's a smallish feature, you don't usually sweat app review. It's obviously not a perfect system.
I'm pretty sure that's what Fortnite did right before they got kicked out of the App Store.
Before my company implemented IAP in a few of our apps we did the same thing, but decided the risk was too great. Getting rejected is one thing, but getting your account terminated is obviously a much bigger deal.
That said "Bugfixes and performance improvements" is as useless as it gets ...