So pre 1980 magazines are permitted but not post 1980. Makes me wonder what happened in 1980 that made these magazines so risky?

Nothing explicit happened, it's just that magazines pre-1980 qualify as antiques, and therefore one can purchase such magazines purely out of legitimate historical interest and preservation - eBay won't buy the 'just read it for the articles' excuse, but they do accept 'in the interest of preserving national heritage'.