I thought it was just the Amiga. Like, Macs had square pixels, to my recollection, both 512x342 and 640x480. I had forgotten that VGA was 320x200 not 320x240, but PCs also had 640x480, so it seems like square or not square depended on the mode.
Strictly speaking, I don't think square pixels were a characteristic of the monitor. If you looked closely there were little dots on a shadow mask monitor considerably smaller than the average pixel, or horizontal gaps between the wires on a trinitron.
Source: lived through those times...