In revision 1554, the src\3rdParty directory was deleted. It was added when we experimentally integrated the Dr. C# addin, but had to remove it later on because it wasn't maintained. What stayed around till that revision was the directory where it lived.
Last week, I had a discussion about repopulating that directory with a third party addin. Various good reasons why to add that specific addin to the distribution exist, but in the course of the discussion (which really forced me to rethink that third party source directory "feature") I found out that having tightly integrated third party addins in our codebase / distribution is no good idea:
For these reasons, we decided to remove that directory and continue to stick with our policy: our repository contains only our code (exceptions: non-addin libraries and build tools), and our distribution of #develop only contains addins that are maintained by us. This way we avoid those sticky issues.