There is nothing wrong with writing two lines of code. And two lines of code may be more readable than one line of code. New method or parameter has a cost -- core developers need to maintain it, update documentation and tests if they conflict with new features, user will need to learn about it, remember differences between Python versions and implementations of different Path-like objects, it can be only used after dropping support of older Python versions, and it may be a source of common mistakes.

If you think that it is so important to use a single call for appending to a file, you can create a module which provides functions append_text() and append_bytes(). They could be used even in Python versions older than 3.11.