I've been using proportional fonts for coding for years. Nobody notices when looking at my screen or when doing a screen sharing session. The few times I talk about it everybody agrees that it's easier to read. I'm not surprised, because nobody formats books, magazines, newspapers, web sites with monospaced fonts. The only problem is that it's impossible to align code on successive lines to the right of an equal sign. Not a big deal. It's also pretty impossible to write ASCII diagrams in comments. AFAIK there is no editor that can help with that, unless one wants to code inside a word processor.
ASCII diagrams in comments/docstrings are doable if your editor supports custom faces. For example `M-x list-faces-display` on emacs tells me that (among other things) I can customize:
- font-lock-comment-face
- font-lock-doc-face
- org-table (also depends on alignment)
I'll try Input Sans.