Yes, there are specs on how much cord can be left showing on a tire before it is considered unairworthy. But why risk your aircraft and potentially a flat tire while landing with a tire this bald? With any water on the runway, it will certainly hydroplane.
The tight-fitting wheel pants on many amateur-built aircraft today do make it harder to check the condition of the tire. However, it’s really not that hard to check it while rolling the aircraft forward or backward and having someone else look at it for you. Or put some water on the ground and roll through it. The tire marks left on the ground will help you determine the condition of the tire all by yourself.