Thanks to all of Valve's effort with Proton, Steam Deck and their funding of people working on various other bits of Linux code like GPU drivers - the Linux share on Steam as of March 2024 bounced back to a near multi-year high.
I don’t know that Steam doesn’t record it anyway, but sharing OS is explicitly part of the data they show you that they will take if you agree to the Steam survey, which certainly implies they’re not doing so if you don’t consent.
The website situation is (arguably) different, as a User Agent is something your browser is giving the website on your behalf (in theory). From the website’s perspective, you are voluntarily sharing that information and any issues with that are between you and your browser.
Consent has no relationship with technical ease.
I don’t know that Steam doesn’t record it anyway, but sharing OS is explicitly part of the data they show you that they will take if you agree to the Steam survey, which certainly implies they’re not doing so if you don’t consent.
The website situation is (arguably) different, as a User Agent is something your browser is giving the website on your behalf (in theory). From the website’s perspective, you are voluntarily sharing that information and any issues with that are between you and your browser.