That property should still exist, but it might possibly be ReadOnly for some reason. The regular name of the PropertySet is "Design Tracking Properties" (or "Project", if using iLogic snippet), and the Property name is "Categories". It is at Index 36, and its PropID is 56. The online documentation for that property says that it is ReadOnly, had no user interface way of accessing it, and will usually contain a String type value containing XML related data. I have not used it for anything myself, but have seen several other similar posts about folks using it to get rid of a document's association with Content Center, Frame Generator, Tube & Pipe related add-ins before, so it must not always be ReadOnly. They very likely need to update their online help documentation. Attached is a PDF of an Excel spreadsheet I created for mapping out and documenting all the standard, and not so standard, but common iProperties. I'm not saying that this is the only way to accomplish what you are trying to do...just saying that the iProperty should still be there, because it is a standard one that is present in all documents, within the third PropertySet. Not all standard properties will be useful in all types of documents though, because some of them are document type specific, and may either be empty, or contain a value that is not useful for much.
Wesley Crihfield

(Not an Autodesk Employee)