I think that the CAT 6 spec is actually unshielded so shielded ones do not conform.
I like this unshielded German cable. Their white paper has things to say about conformity with the standard:
Nevertheless I am currently using Melco ethernet cables with my Melco NAS /Vivaldi. These are shielded but, in this particular role at least, I marginally prefer them subjectively over the Meicord. Same price.