They're updating their backlog of games to support the Steam Deck.
L4D2 just now got an update in the same vein too, as well as Tyler McVicker reporting on a similar update coming soon to Half-Life 2, with footage from the beta branch to boot. I'm not sure about any earlier titles like Half-Life 1, but it seems they're committing to updating their source games for this.
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I can confirm that they have updated Portal 2!
They're updating their backlog of games to support the Steam Deck. L4D2 just now got an update in the same vein too, as well as Tyler McVicker reporting on a similar update coming soon to Half-Life 2, with footage from the beta branch to boot. I'm not sure about any earlier titles like Half-Life 1, but it seems they're committing to updating their source games for this.