Locally hatched ground plane to increase differential impedance
I think the hatched ground plane is more technological technique. It is used for better adhesion of layers on a flexible PCB or to reduce thermal dissipation in manual soldering.
Sometimes a single ground plane cutout is used to optimizing impedance when a signal meets a discontinuity.
It seems that impedance tunung with multiple cutouts is not a good and widespread practice because of electromagnetic compatibility. The HF-line with ground cutouts strongly resembles a slot antenna.