Indeed. Any online store can go under or enshittify.

If you want to stay safe with GOG, download the offline installers for your games and archive them. If you don’t you’re running the risk of losing some or all of it.

Fortunatelly GOG has that possibility as standard, Steam does not make that option available - at best you can copy and zip existing installations of Steam games and hope for the best, a which is not official supported.

The point being that Steam could make something like that available, at least for games whose developers/publishers are willing and mark that as a feature in the game page in their store, but they have chosen not to.