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Don't think there was any actual armour prohibition, but they certainly ordinarily didn't seem to wear any. A few online articles written by modern Sikhs claim iron breastplates and mail shirts were part of their garb in time of war, along with the more well known accoutrements. Unfortunately none of the few books I have on the subject mention armour at all. If they wore it I think you probably don't see it depicted very often simply because most of them couldn't afford it.For what it's worth there is a Sikh fellow in the British West Midlands who claims to be the last one (though that is debatable). He teaches the "lost" Sikh martial arts, and has no problem at all wearing armour whenever the media show up to interview and photograph him. Take that with a pinch of salt though, as obviously he's not an authentic 19th Century Akali Nihang, he's some bloke from Wolverhampton.