When reading the rules about advertising for others it lists this:

CoC 1.10 Do not post content or links relating to other user's accounts, sales, raffles, lions etc.

-Do not solicit other users to advertise on your behalf.

-This excludes linking specific lions or threads for solely informative reasons, for example to share a game guide or a new mutated lion that was born. Excessive linking of the same topic will be considered advertising.

-This excludes clan owners using their clan thread to advertise clan members' raffles etc at the member's request.

-This excludes "stud collaboration" threads where every stud listed in the compilation has a corresponding entry in the thread itself from the owner of the stud.

Yet nowhere does it list anything about reverse studding, so there's no clear definitive answer as to whether or not reverse studding collaboration threads are allowed, and it leaves people unsure as to if what they're doing is against the rules. I think it would do good if the rules were updated to include the allowance of reverse stud collaborations, so that it can actually be seen and read as allowed rather than just assumed, and would clear up any confusion some people may have about it.