This is a real problem, but it has gotten quite silly at this point. A user agent can reject cookies by merely not doing anything with them. A user agent can

This is a real problem, but it has gotten quite silly at this point. A user agent can reject cookies by merely not doing anything with them. A user agent can signal that it accepts cookies by just retaining them. You might argue that we could save a pittance in bandwidth by eliding the Set-Cookie header entirely. However, the potential savings here is a drop in the bucket compared to the ocean of javascript to handle the consent banners.