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.