No it doesn't.

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...

Warframe, one of the most active userbase game on Steam, literally has a dedicated page just to purchase a platinum coin in their own website [1]

[1]:https://www.warframe.com/buyplatinum

Steam does have rules about it, but they are not strictly enforced. If your game have it's own backend / website then you can freely bill people there after you onboarded them via Steam.

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.