There are many aspect ratios of fixed width fonts (see https://int10h.org/oldschool-pc-fonts/fontlist/), from square to rectangular, and this one is more than twice as tall as wide

Is there any info or research on which width/height ratio of a fixed width font is the best for readability?

Don't forget that a lot of old PC graphics modes weren't 1:1. 640x200 was a thing, for example - the pixel ratio is 3.2:1, but it gets vertically stretched to a standard 4:3. An 8x8 font wouldn't look square in that graphics mode.