With each major release, the packages' structure can change, causing extension package to fail due to a missing package reference. The opposite can also happen, where a package reference is no longer required (as the referenced element has moved to a different package).

It would be useful to give a message in the project build output listing missing package references, and also to suggest a reference is removed.

Adding references is merely a time consuming exercise as it won't build until they are added, but there isn't any help removing unnecessary references.