It wouldn't be universal, what about getting rid of 4(1)a (which also doesn't require states to recognise conscientious objectors!) in the ECHR? Conscription was given a specific exemption from the anti-slavery provision in article 4. The ECHR has a few clearly stated exemptions for things that in retrospect seem odd e.g. the death penalty (later removed by a protocol). It would be no use when a government claims there is a war or national emergency, as the ECHR allows governments to ignore it if there is some kind of war or national emergency, even if that national emergency or war isn't really a national emergency or war. Also, what if the UDHR was binding? There have been arguments advanced that conscription is against articles 4, 20.2 and 23.1, and they look correct on the face of it.