I really like the in-app camera in Lightroom for iOS/iPadOS. The only thing I really dislike and wish I could get rid off are this massive black overlay bars over the viewfinder window at the top an bottom. It's often not possible to take a well balanced photo, because you have to guess whats behind this black bars.
At the web-tutorial under https://helpx.adobe.com/lightroom-cc/using/capture-photos-mobile-ios.html there is a passage ("Hide user interface controls") describing a way to hide the in-app user interface by swiping down over the viewfinder sceen, but I don't get it to work - it simply does not work (at least not at my iPhone 12 mini / iOS 15 nor at my iPad Pro 11" / iPadOS 15). Did I missunderstood this function or what's up here?
I'm afraid you found a mistake on our help page. I will let our Help team know that the hide controls functionality you mentioned from that page has been deprecated and is no longer available.
However, the capture viewfinder should not be obscurring anything as you described. By default, the capture aspect ratio is 4:3, and the entirety of this view is displayed in the LR Camera. The black bars where the camera controls sit are not blocking any pixels that will show in the final image.
What a pity Adobe droped this feature, I would have used it a lot!
Than take it as a feature request to implement this swipe gesture again - or alternatively getting rid of this massive black bars altogether (maybe as option in settings or at least with 50% more transparency). I do not believe, that any professional photographer accepts, that he hasn't full control over his entire viewfinder...