In zmq_setsockopts API v4.1 it says:

ZMQ_XPUB_VERBOSE: provide all subscription messages on XPUB sockets
Sets the XPUB socket behavior on new subscriptions and unsubscriptions. A value of 0 is the default and passes only new subscription messages to upstream. A value of 1 passes all subscription messages upstream.

In zmq_setsockopts API v4.2 it says:

ZMQ_XPUB_VERBOSE: pass subscribe messages on XPUB socket
Sets the XPUB socket behaviour on new subscriptions. If enabled, the socket passes all subscribe messages to the caller. If disabled, these are not visible to the caller. The default is 0 (disabled).

Obviously, this breaks the backward compatibility