Does it matter if differential pairs have no reference plane or cross reference planes?
A reference plane is not a requirement but most designs use one.
Normally some of the return current will flow on the reference plane depending on how closely the differential pair is spaced. The reference plane also plays a role in the impedance of the differential pair. Achieving 100 ohms differential impedance on a PCB might be difficult without one. Reference planes provide other benefits like EMI reduction and isolation from other signals.