Steam's new disclaimer reminds everyone that you don't actually own your games, GOG moves in for the killshot: Its offline installers 'cannot be taken away from you'
It technically still exists in the game properties -> installed files tab, but it doesn't really work. The backup files you get require you to be online to meaningfully restore and will trigger a patch to the latest game version.
Practically speaking it's better to just make a copy of the game install directory manually, gives you a better chance of things working (even though most games require some kind of external tooling for that).
For current exports, it's some custom .csm/.csd file combo. Not sure if there's any tools for working with it, seems like it'd be more annoying than just using a normal archive format either way.
It had a way of packing a game into a CD/DVD when it launched. I used it all of two times. It was slow as fuck. If it still has it, as another commenter suggests, I don't know how to access it.