"Note: From Microsoft
In .NET 4 the Workflow Team released an all new Workflow engine in the System.Activities namespace. With the release of .NET 4.5 Beta we are marking the types in the “WF 3” System.Workflow namespace as obsolete. The following assemblies have one or more public types that will be deprecated:
As a result, customers who are using the deprecated WF 3 APIs will encounter build warnings with a message similar to “ Warning BC40000: X is obsolete: ‘WF 3 types are deprecated. Please use WF 4 instead.’ ” We will remove the types from the .NET Framework in a future release, but we have not yet determined that timeframe (not in 4.5). This current step allows us to communicate our direction to our customers and allow them plenty of time to move to the new WF4 model. ...
Given the complete re-write that was WF 4, and the break from v3 that it was, this isn't too surprising. This should give any existing WF v3 users a few years to move off (as, given this move, I'd bet WF v3 will be nuked in .Net v5). So if you're using WF v3, now's the time to at least start planning for you move to v4+