Just did a GOG survey that focused on the idea of a paid membership option on GOG. Seems they're determining what people would be willing to pay extra for. Some of the options were
- a tool for backing up offline installers
- ability to install previous versions of a game
- extra insight into the preservation work they're doing.
- voting rights on games to bring into the preservation program.
And others that I can't remember.
Ah, seems you're partially correct - steam has a command for downloading a specific depot version. You need to know the specific ID to download, and notably games can use multiple depots to form the game files, but I thought you needed to use something like SteamCMD or DepotDownloader for that.
I'm still upholding the fact that it's not a "proper" feature, while I appreciate having those kind of utilities put in the user's control, this isn't something most people could figure out themselves.