It's sounding like California is going to require browser manufacturers to support the GPC signal. The privacy movement in California has a lot of political power and backing; it's pretty likely this will change in the next couple years.
California's "Opt Me Out Act" (AB 566) requires that by January 1, 2027, internet browsers must provide a built-in, easy-to-use setting that allows users to send an opt-out preference signal, such as Global Privacy Control.