Comment by Lalabadie

iA Writer has Mono, Duo and Quattro fonts, with the latter two being almost monospaced. They concede some size variations for specific characters (Duo has 150% width characters, Quattro also uses 50% and 75% for narrow characters).

It's a fun subtle adaptation to keep close to the typewriter-like experience of the app.

https://ia.net/topics/a-typographic-christmas