The reason we don’t see square traces in most PCB designs is simply due to aesthetic convention. If it looks wrong, it is wrong. There is no specific reason why every circuit board design should shy away from 90-degree angles on traces, it’s just something that you shouldn’t do because it looks wrong.
Because 90-degree turn has a higher VSWR when you use two 45-degre turns and ‘acid trap’ too …
you shouldn’t do because you shouldn’t