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.
That is absolutely not a free feature on Steam. Some publishers like Paradox leave old versions as 'beta' branches to allow us to reinstall them, but Steam as a whole is very against you playing anything but the latest version.
You cannot instruct Steam to not update a game. When you launch a game, Steam will update regardless, unless you have gone offline, or you launch it in a way that bypasses the Steam client. If you ever forget to go offline before launching a game, Steam will forcibly update it