Really? Yes, it's tested. It's not an assumption. The Product Categories can be deleted from the UI without issue.
The UI actually uses https://...Services/ReliableCommunicationManager.svc/ProcessMessages?cmp=abc&lng=en-us& instead of making a restful call, which I was hoping to see.
For whatever it's worth, the UI does prompt and ask if the user wants to check if the category is referenced by other documents. I'm wondering if that has anything to do with it, perhaps there is another method/resource that needs to be called or pass something in the body stating that the delete operation is "confirmed", however, the documentation that I can find doesn't say anything of the sort. And again, using both Postman and an OData client in c# return success statueses from the server, although it's not deleted.