Is a snubber for transformer primary winding necessary?
In a conventional transformer, the primary and secondary are strongly coupled.

If the switch opens when there is a large flux in the core, when the flux collapses it induces a voltage in both primary and secondary. The secondary will typically be loaded with a rectifier and large capacitor, which will absorb the energy and clamp the voltage (of both windings) to safe levels.

There is an element of transformer inductance that is not loaded by the secondary, and that's the leakage inductance. This tends to be very small in power transformers, and stores little energy, which is easily handled by an adequate rating for the mains switch.