http://modpython.org/live/current/doc-html/pythonapi.html#mu...
EDIT: And it looks like I had subinterpreters in the first released version in May 2000, so the initial git (formerly SVN) commit already had them https://github.com/grisha/mod_python/blob/9b211b7e8a65f1af4b...
EDIT2: Just noticed this comment:
* Nov 1998 - support for multiple interpreters introduced.