It's interesting that it's been removed, because it did provide a solution for my specific case (moving views with scoped region managers between regions).
What the MoveFrom function gave us was effectively a limited way to insert a view which already has a region manager attached to it. This is all I really need to achieve. I'd favor a more generic function - i.e. an AddViewWithRegionManager() or similar.
Previously I was using my workaround to the RegionManagerRegistrationBehavior (mentioned in the thread you linked to above) to prevent the re-assignment of region managers which happens when a view is reinserted from blowing up. However, it's much cleaner
in operation if we don't have to re-create a region manager at all, when the view is put back in, as there are quite a lot of knock on effects in other extensions I have to Prism when this happens.
I'm happy to write extensions to Prism, but when we start modifying the source code it becomes much harder to upgrade in the future. Honestly, I'm debating whether to splurge out some Reflection code to invoke Region.InnerAdd()...