This was the original proposed idea in the PEP (750), but it changed overtime. There is a section in the PEP to explain why it changed to t-strings if you are interested.
PEP 638 has always seemed to me like something of a generalization of the idea. But that really feels to me like a 4.0 feature, or rather something that needs to be designed for from the beginning. (Which is why I've done a bit of that in my own mind...)