Not sure, there aren’t many reports so it’s hard to say. I know at least the ROG Ally version has its own service which sets the TDP so it’s probably not affected.
As for the Steam Deck, if you’re running this on an actual deck it’s not really a concern because 15W is the Deck’s default TDP. And Bazzite-Deck is Steam-Deck-first distro, so you’re still really better off using Bazzite.
Also, to clarify, this is a case of Steam (Gamescope) itself changing the TDP, so it’s not a bug introduced by their devs.