Python’s new t-strings

If it's not a language feature, there's always a risk of fragmentation. Some people won't use it because it adds another dependency, that means fewer programmers will be familiar with it. Others will come up with their own, slightly incompatible implementation. See for example Perl and its various Object Orientation frameworks (Moose, Mouse, Moo, Mo, M, Mojolicious comes with its own...)

Other languages have a policy of prototyping such things out of core, and only adding it to the core language if it gains traction. Of course that works better if the language has a mechanism for extending the syntax out of core.

putting it in the standard library would handle those issues.