If the "just a string" is html, sql, python, etc., the programmer is making a mistake as string interpolation shouldn't be used for those things in the first place, and I really hope < 80% cases of "just a string" are among those.

"just a string" where the string really is just a string is really, really common though. Python is the go-to language for a lot of people (including me) if they just want to bash together a one-time-use script that will never see input from other people, and f-strings are incredibly useful in such cases for generating filenames, debugging messages, etc.