The new prototype editor is a great new feature in IDA 9.0 , even if the UI is a bit rough for the moment, it took me a few minute to figure out how it worked.
But it is not available from the decompiler (pressing Y in the decompiler gives the old “Please enter a string / Please enter a type declaration” dialog instead of the new “Edit function prototype” one).
I don’t see why it would not be there, so I guess this is an oversight
Cheers