Variable width fonts are generally more readable, which is why they’re generally used for prose.
Fixed-width fonts allow for easy tabular rendering in a text editor. Some programming styles use this heavily but it’s not universal. Particularly, if you rely on line-by-line blame in source control, alignment changes can make it harder to find the commit where the most recent semantic change happened.
Traditional fonts even had non-lining figures, with varying heights for easier readability. Modern-style figures were only really used for tabular display, or with content written in uppercase/title-case. (Of course, it's hard to support this kind of complex text styling when editing plain-text files that are going to be fed to a compiler, or some other text-processing step.)
But source code is very different from prose. Are there any studies on fixed-width vs. proportional readability specifically when it comes to programming?
Fixed-width fonts allow for easy tabular rendering in a text editor. Some programming styles use this heavily but it’s not universal. Particularly, if you rely on line-by-line blame in source control, alignment changes can make it harder to find the commit where the most recent semantic change happened.