Valve is making some changes to Steam's automatic game update system in order to regulate bandwidth usage as the ongoing coronavirus pandemic drives unprecedented numbers of users to the platform.
Ordinarily, Steam automatically queues updates for any game in your library. However, as of this week, only games that you've played in the past three days will update automatically. Any missing updates will begin if you manually try to play a game, and you'll still be able to initiate any updates through the download manager or your library, they just won't come automatically. Note that Valve isn't actually throttling download speeds for the updates themselves, like PlayStation has for Europe and the US. Some downloads won't start automatically, but when they do start, they'll be as fast as usual.
Valve says it's "looking into additional solutions to help on our side." In the meantime, the company encourages Steam users to customize their download settings to suit their needs. You have the option of setting update windows, pausing updates on less-played games, and manually throttling your own download speed, either because Steam is eating into your other online activities or because you want to preserve bandwidth for your area. Valve also advises players to avoid uninstalling games if they expect to play them in the near future, that way they don't have to redownload the whole thing again and use up that precious internet juice.
Companies like Valve are making infrastructure changes to accommodate increased traffic driven by all the people stuck at home due to coronavirus, and the games industry as a whole has also joined together to support coronavirus relief efforts.