If you don't want to pay Steam's cut, or follow their restrictions you have plenty of other options. You are not restricted in the sense you are with iPhone + App Store.
I guess the question is does Mobile gaming cover handheld game device like Switch? If so, it may impact Steam Deck as well.
https://www.polygon.com/22579033/valve-steam-deck-handheld-e...
This has more interesting implications for Nintendo, Sony and Microsoft consoles.
Valve even prevents developers from linking to their own website, if said website includes ways to buy content independent of Steam payment.
See Rimworld. https://store.steampowered.com/app/294100/RimWorld/
Sidebar has link to visit the website. https://rimworldgame.com/ You can buy directly with Credit Card or Paypal.
Fantasy Ground is the same way: https://store.steampowered.com/app/1196310/Fantasy_Grounds_U...
IAPs are different: here Valve actually require you to process payments through steam when game is running via Steam. But even in this case a lot of games can be launched independently after installation through Steam.
Also on top of this Valve already decreased it's comission from 30% for large publishers.