This will needlessly break code which until now accepts both kinds of sockets.

By the way, socket.shutdown() doesn't specify that *only* one direction is shut down when using SHUT_RD or SHUT_WR; what is guaranteed is that *at least* the given direction will shut down.  But there may be socket types where unidirectional shutdown is not supported and both directions will be shut down.  This is (approximately) what SSLSocket does -- though the SSL unwrapping part is a bit unintuitive as well.