Hi Andreas,
I believe the Prism library was moved to GitHub here: https://github.com/PrismLibrary/Prism
Prism is still used by many WPF developers and it is now supported by the open source community as part of the .NET Foundation. If your app already uses it, it's fine to stick with it.
As for the RibbonRegionAdapter, I don't believe we define that. Perhaps it's something your developers created?