A string prefix would be a large language change that would need to go through Python-Ideas, a PEP and Steering Council approval. Let's not go there :-)

A new string method is a comparatively small new feature that probably won't need a PEP or Steering Council approval.

A compile-time optimization is an implementation feature, not a language feature. Whether devs want to follow up with more string optimizations like 'spam'.upper() is entirely up to them.

Backwards compatibility implies that textwrap.dedent is not going away any time soon, but it should probably become a thin wrapper around str.dedent at some point.