@thatprivacyguy hi Alex. IANAL, but I don't really see what adblock detection has to do with accessing information stored on a device. The way they implemented this is likely giving unique ad IDs, and then checking if connections are being made to those IDs, when you load the page. If not, then it's obvious what is happening.
Look, I dislike this YouTube move as much as you, but I really don't think GDPR is appropriate to handle this. This does seem more like a case of a business rejecting customers by not providing them with the requested service at their discretion. This, by the way is fully legal, as long as it is not a targeted campaign against protected minorities (women, LGBT, refugees), which very clearly isn't the case.
Can you elaborate further on why you think GDPR applies here?