Sometimes it's not worth the hassle on either side. If this is added to the standard library and I write code that uses it, my code won't be backwards compatible with older versions of Python. So I'll either have to not support older Python versions or use an alternative implementation. If this is on pypi then that's not an issue. Not everything useful should be in the standard library.

If I had come across a need for this, I'd have just used unique_everseen(permuations(...)) until performance became an issue.