"raw" and "byte" are one syllable names, and thus as easy and more meaningful to say than "r-string" or "b-string".  "unicode string" is more descriptive and not much longer, but "u-string" does occasionally get used, though mostly in a python3 context where you want to differentiate it from normal unadorned unicode strings.  If you want to replace 'f-string' in the vernacular, you need a one or two syllable word that is descriptive.  The only real candidate is "format string", and we already use that for the older types of format strings.  If those were to fall out of use, I'd expect "format string" to come to mean what f-string does now.  But by the time that happens, f-string will be more entrenched than it is now.  We could try for "format expression", but that is more syllables, and faces headwind because the f prefix obviously makes it a "string"-like object, and that's how we think about it: a string with expressions inside it, instead of as an expression itself.

In summary, I think we're stuck with f-string.  It's a term of art: a short expression that encapsulates a non-trivial concept for which there are no precise existing words.