Proportional fonts break code navigation, moving vertically is much more important in code than normal text. Also it's harder to line things up, and alignment is a good cue when scanning code.
I would argue that most use of column alignment in source code hampers readability and maintainability.
There are some exceptions, of course. If you are writing out hard coded matrices, you may want to line up the numbers.
But how many of us really do that day to day?
Many modern formatters such as Black for Python and rustfmt for Rust avoid column alignment and prefer indentation instead. Then the code is equally easy to read and navigate in any font of choice, proportional or monospaced.
