Saw this today - seems new?
Yes it is new. And seems like it could be very interesting alternative to Enterprise licenses, although you would get the delay of app store reviews. But this is NOT intended for beta versions or a Test Flight replacement. Rather it is intended for “final” or stable releases.
And I believe you have to apply to have an app qualify for unlisted distribution and justify the reasons. I think it remains to be seen how lenient Apple will be in that approval process.
And it’s not protected in any way. Anyone with the link can find the app and download it.
Don’t see the issue really - am sure the link will be obscure enough to make it impossible to find.