Sometimes UE gets IP address of P-CSCF as part of attach procedure, it is a kind of notification from network to UE that it has IMS service support. I also observed even after the attach procedure, when UE want to initiates IMS signalling with IMS domain nodes it should have P-CSCF IP address since signalling message flows through P-CSCF, I-CSCF, S-CSCF nodes.
For that purpose, UE does P-CSCF discovery by using DNS queries. As far as I know, network does not broadcast saying "it does not support IMS service". However, network will send or not P-CSCF address to UE, it depends on received PCO "Protocol Configuration Options" from UE in the attach request message.
If UE has requested for primary DNS ip address, secondary DNS ip address etc. PGW includes responses for all the received request.