Does an infinite AC bus which gets a load step experience any frequency sag?
In a mechanical, spinning, voltage source, energy is stored in the rotational inertia of the alternators and turbines. It's this energy that is used to respond to step changes in load, and so yes, the rotational speed will drop until the mechanical power input can be increased to compensate.

In an infinite bus, only an infinitesimal change in average frequency is needed to release the energy for the step load change. However, the local machine(s) will still need to slow down for a moment, to create the phase shift that's needed to import energy from the more remote parts of the grid.

If the AC bus is entirely inverter driven, then what happens to the frequency depends on how the inverter has been programmed. It could respond to any load with the same frequency. 

Or it could, if it's been designed to 'look like' a conventional grid, drop the frequency. Why? Some limited capacity supplies, so a small diesel or wind turbine on a small island for instance, may use automatic frequency sensitive load shedding switches on 'second class' loads like water heaters, to maintain supplies to the more important loads when the wind drops. This means an inverter could be used, and still work with the existing load shedding arrangements.