From a suggestion on the Autodesk Idea Board:
When you pin something there should be an option to write something in a “comments” field. If someone unpins that thing then the comment you wrote would pop up like a warning dialogue. This way they would know why the thing was pinned in the first place.
I’ve created a new free tool that uses the DocumentChanged event and an External Event to prompt the user for a reason when an object is pinned. When the object is unpinned, it shows the username and reason.
Like with all the Boost Your BIM Terrific Tools, you can find the source code & installer at https://bitbucket.org/BoostYourBIM/boostyourbimterrifictools/src/master/