Valve is currently testing out a new Game Updates option, giving users "more control over how Steam delivers updates" to their installed games. This new option is available for testing in the latest Steam Client Beta, detailed in a new Stean News post as a means "to strike a balance" between having their most recently played games ready to launch and effectively using their bandwidth for those with internet caps.
This option will let users prioritize when to install game updates with a new Default option, which allows them to tailor some exceptions for individual games, as recently played games will install "shortly after they are released." The latter is extremely important for exceedingly large updates and games that they haven't launched in a while, as they can toggle settings to withhold updating games "to bundle multiple updates together" or update them when users are asleep.
The new Game Update option is available from the beta Steam Client Settings drop-down tab within the "Downloads" option, where this new option can be found. Users can set the Default option to allow "Steam decide when to update" the game and give users some factors or exceptions like when it was last played, waiting until they've launched the game, bandwidth availability, and other filters.
Users can further optimize these settings per game from their Library, which overrides the global settings made from the Settings with more specific factors to enable. They can select the game and click the Gear icon, Properties, and then the Updates option before adjusting their update options, which can be managed from the "Downloads" option within the beta Steam Client Settings.
Remember that there are other settings available that users can explore from the Settings menu, including controlling download speeds and limiting downloads while playing or streaming games. The timeline on when this setting will be available globally, but for now, those with access to the beta Steam Client can try them out immediately.