Purpose of bare copper perimiters on HF PCBs
Think of your PCB as parallel plate waveguide, with the top and bottom ground planes acting as parallel plates.

Now, since you want waves neither exiting nor entering this waveguide, you'd try to build a "wall" around them – or more, a fence, which the small vias around the edge do.

The fact that the ground plane is exposed around the corners (i.e. no solder mask) might point to the PCB potentially being mounted in a conductive, hence shielding if grounded, enclosure.

Rounded corners are nicer to handle, and don't break off. Also, æsthetics.