If anyone is interested,
I found this webpage: https://mackuba.eu/2019/12/03/new-stuff-from-wwdc-2019/#comment1
I asked this question:
Could you be specific about where during WWDC 2019 they said something about CNChangeHistoryFetchRequest and CNChangeHistoryEvent for the Contacts Framework?
I got this answer:
@ShinehahGnolaum Hey! These lists are combined from the things mentioned in the talks, but also from looking through the documentation e.g. at https://developer.apple.com/ios/, release notes, and just simply API reference (I use this page http://codeworkshop.net/objc-diff/sdkdiffs/ to see what new classes/methods were added in some interesting places). So some things listed (a large part of them actually) might not have been mentioned anywhere in the talks and I've just found them elsewhere. It's not necessarily "things mentioned at WWDC", but "things released in the betas at WWDC".
It looks like these two you've mentioned are things like that, because I don't see them anywhere in any talk notes or transcriptions. Which is unfortunate, because I see they still don't have a proper documentation… which is probably why you're asking. I think the only piece of information that can help you somewhat is the ObjC header CNChangeHistoryFetchRequest.h in the SDK - you can access it through the "Open Quickly" (Cmd+Shift+O) in Xcode. There are some code comments there that aren't rendered in the web documentation. Not much in CNChangeHistoryEvent.h though.