I think GOG might be an exception to this, where they will never revoke access to the games you previously bought, but I am not 100% sure of their policies.
They are, when you buy a game from gog they send you the installation files. You install and run it with your own hardware whereas with Steam and other digital gaming companies you are just getting access to the game on their servers. By sending you the installers and letting you play independently from their servers gog gives up the ability to lock you out. It’s the primary reason that they should be the first choice for where to buy a digital version of a game. The upside is that it’s the closest you can get to actually owning a digital copy of a game; the downside is that playing on another device requires that you transfer files and reinstall rather than just logging into a remote server.
They are, when you buy a game from gog they send you the installation files. You install and run it with your own hardware whereas with Steam and other digital gaming companies you are just getting access to the game on their servers. By sending you the installers and letting you play independently from their servers gog gives up the ability to lock you out. It’s the primary reason that they should be the first choice for where to buy a digital version of a game. The upside is that it’s the closest you can get to actually owning a digital copy of a game; the downside is that playing on another device requires that you transfer files and reinstall rather than just logging into a remote server.