Not really, there's also navigator.cookiesEnabled now. And even if there wasn't, attempting to store a cookie with a dummy value just to check if cookies are e

Not really, there's also navigator.cookiesEnabled now. And even if there wasn't, attempting to store a cookie with a dummy value just to check if cookies are enabled does not break the GDPR for three reasons: first, if it's a dummy value it's not really personally identifiable information. Second, it's a functional cookie required for the site to work. And third, the site can just delete it afterwards the checking. No consent required in this case.