steam game can be played on Windows, MacOS, or Linux without restriction provided it's ported or works with Proton.

Usually you would be playing a windows x86 build via Proton on an x86 (or emulated x86) system, but AFAIK Proton doesn't have a backend for iOS/Android (which are ARM), and steam won't even run on an ARM linux ubuntu. Devs most likely would refuse this because converting a game to Android/iOS is a whole new build, they might not have been using an engine like Unity/Godot that simplifies adding Android/iOS as build targets, they might not be willing to redesign UI elements for a platform that's touchscreen-first, they might not want to hire additional QA/play testers for mobile devices, and if they are willing to do all of that, they might not be willing to give it for free for users who bought the x86 windows build, they might even want to make the game's revenue model ad-driven or pay-to-win.

But you can ignore all that and buy a steam-deck like device, or download the "Steam Link" app and stream a game from Steam to your mobile device.