If they are talking about Java APIs, it's not that these APIs are hidden, it's some functions that were never exposed are being misused by apps using reflection

If they are talking about Java APIs, it's not that these APIs are hidden, it's some functions that were never exposed are being misused by apps using reflection. Some of this functionality has been allowed to be used by apps because there was or is not a good replacement, or a large number of important apps were using that functionality. The trick is that the use of those functions is gated by the Android API you are targeting, so if you target the latest API, you can't use the "hidden APIs" any more. I'm not sure what this is bypassing, the API check or maybe some of the new reflection checks. Doing this is a recipe for disaster, because your app will stop working in a future release as the "hidden APIs" are removed, or the methods to access the "hidden APIs" are removed. You are guaranteeing that your app will fail soon. So don't ever use this.