Here it is. https://mail.python.org/pipermail/python-3000/2008-November/015344.html

On Sat, Nov 22, 2008 at 06:29, Barry Warsaw <barry at python.org> wrote:
>
> On Nov 22, 2008, at 4:05 AM, Martin v. Löwis wrote:
>
>> I just noticed that the Python 3 C API still contains PY_SSIZE_T_CLEAN.
>>
>> This macro was a transition mechanism, to allow extensions to use
>> Py_ssize_t in PyArg_ParseTuple, while allowing other module continue
>> to use int.
>>
>> In Python 3, I would like the mechanism, making Py_ssize_t the only
>> valid data type for size in, say, s# parsers.
>>
>> Is it ok to still change that?
>
> Given that we just released the last planned candidate, I'd say it was too
> late to change this for Python 3.0.
>

But we can at least document that the macro is a gone as soon as 3.0
final is out the door.

-Brett