I liked this idea. It would enforce the types of returned values from functions and methods, just like the parameter checking they added a few versions ago.
A bug was found during the voting period that will require enough changes to how the RFC works that voting has been cancelled.
Apparently the problem lies in that the PHP compiler only uses a single pass and allegedly, the compiler does not know about classes (i.e. new types) before they have been defined, which means it’s tricky to type check. Since there was so much support with this RFC, I am sure the next one will work just fine too.