I thought of that game as well, but I never played it or even knew about it at the time. I also forgot about the connection to Slack. What I DO remember is that they released the game art with a CC0 license after shutting down the game. The site seems to just redirect to a 404 on slack.com now.

https://web.archive.org/web/20161124184152/http://www.glitch...

https://archive.org/details/glitch-public-domain-game-art