As you are no doubt aware, .NET 4.5 shipped in October.
With the release of 4.5, our team has deprecated the original Workflow engine aka WF3 (System.Workflow, etc.). There is a detailed announcement from Jurgen on the team blog.
The types are marked deprecated but have not been removed. Using the types will result in build warnings, but the types may still be used. However, they will be removed in some future release.
I also want to point out that the types related to the WF Rule Engine are not marked as deprecated.
Full details of items marked obsolete in 4.5 can be found here.