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The kernel itself uses a strong copy-left license so *any* Linux distro on Steam Deck would require the same diligence for Valve. The key matter is "modifications". If you are OK with releasing code modifications as source under the same copy-left license, all problems go away and you can charge whatever you wish for distribution with few legal costs. If releasing modifications as source under the same license is not desired, then you have to stay clear of modifications of copy-left and now you're dealing with lawyers protecting/defining your "modifications" to minimize exposure to a license challenge.
In Valve's case, it would not surprise me if they're playing both sides: don't publish modifications but be prepared to do so if/when challenged. If someone can then build a competing "Mist Duck" with their source code, they still make money off the games being played on it....
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