I think you are looking for the concept named “tainting” in other languages.
Guido: No, I wasn't looking for full-fledged taint analysis for finding security vulnerabilities. (We do that with Pysa [1], for example :) )
Format strings, explicit database queries, etc. are places where we simply don't want to allow any non-literals. As Jelle had pointed out, people already write ad hoc tools to check for literalness. I think the concept of arbitrary literal strings gives us a simple, readable way to express our intentions.