Could’ve been long ago

This nomintally existed in 2.x. It was (and nominally still is) spelled string.Template, but unfortunately, you had to implement 90% of it yourself, because it just provides a top-level entry point for overriding the whole interpolation: https://docs.python.org/3/library/string.html#template-st...

Supposedly it is used for i18n, but I have no familiarity with that use case. If t-strings end up being wildly popular, they might plausibly end up absorbing and replacing this use case.