BTW: The API PyUnicode_AsString() is pretty useless by itself - there's
no way to access the size information of the returned string without
again going to the Unicode object.

I'd suggest to remove the API altogether and not only deprecating it.

Furthermore, the API PyUnicode_AsStringAndSize() does not follow the API
signature of PyString_AsStringAndSize() in that it passes back the
pointer to the string as output parameter. That should be changed as
well. Note that PyString_AsStringAndSize() already does this for both
8-bit strings and Unicode, so the special Unicode API is not really
needed at all or you may want to rename PyString_AsStringAndSize() to
PyUnicode_AsStringAndSize().

Finally, since there are many cases where the string buffer contents are
copied to a new buffer, it's probably worthwhile to add a new API which
does the copying straight away and also deals with the overflow cases in
a central place. I'd suggest PyUnicode_AsChar() (with an API like
PyUnicode_AsWideChar()).