house_arrest used to allow direct access to the documents and container of any app. It's used by the Xcode "installed applications" list, to allow you to download and upload container contents. In 8.3 they changed it to only allow VendDocuments access to apps which actually have document sharing enabled in their Info.plist (and VendContainer access for ad-hoc provisioned apps, iirc).

Backup is done via a completely separate service.

iOS releases do tend to arbitrarily change the security policy of these services, but it's not clear that "Rootless" is anything more than business as usual. Guess we'll find out Monday.