The rights owners for pong (Atari?) had it and any mirrors they could find taken down.
A quick search suggests that there are downloadable copies out there. 1.3c was the latest release. Check WayBackMachine from 2007-Sep-13 (https://web.archive.org/web/20070913204145/http://www.plasma...) or older if you want more info, after that the site is an "Atari told us to take this down" message for a while and is currently held by someone else talking about other games.
A quick search suggests that there are downloadable copies out there. 1.3c was the latest release. Check WayBackMachine from 2007-Sep-13 (https://web.archive.org/web/20070913204145/http://www.plasma...) or older if you want more info, after that the site is an "Atari told us to take this down" message for a while and is currently held by someone else talking about other games.