Why exactly is it discouraged to gap the ground plane?
High frequency return currents want to follow the outward currents due to inductance. 

If you force the return currents to take a different path then a couple of bad things happen.


You create a loop that can receive and transmit magnetic interference.
You introduce extra inductance in the signal path which can reduce signal integrity.


Note that digital signals with fast edges can produce strong high frequency spikes even if the switching rate is low.

Note also that the outward path may not always involve just tracks, it may be inside a component. Even if a component has separate analogue and digital power and ground pins there are likely to be some signals crossing over the boundary inside the chip.

OTOH at low frequencies currents take paths determined primerally by resistance. So splitting planes can be a useful technique to influence the path return currents take and avoid shared impedance.

If you have exactly one place where signals cross the mixed-signal boundry then splitting the plane makes a lot of sense, it forces analog return currents to stay on the analog side and digital return currents to stay on the digitial side.

If you have multiple places where signals need to cross the mixed signal boundry (i.e. multiple ADCs, multiple analog switch chips etc) then the benefits of splitting get much more questionable. Each mixed signal chip needs a connection between the two planes but once you put multiple connections between the planes you lose a lot of the benefits of splitting them in the first place.