Doom 2 - what's the story with the "pirated" v1.666 DOOM2.WAD?

As per title.

Does anyone have any details on what exactly made this IWAD "pirated"? When was it released/by whom? What differences does it have from the non-pirated v1.666?

It was leaked from id Software or something like that. The only real difference is the file's date, and that the official patches won't work on it (solely because of the date).

Yep, it was leaked. Therefore no retail owners of Doom2 would be unable to patch.