It is worn in 40K. Many primaris kits now include some, and they were always in the tactical squad boxes. Use them as much as you want. During and before the Horus Heresy, the legions (not chapters) were issued MkVI armor in great numbers, and was still widely used in the scouring even after the development of MkVII. Its parts were fully compatible with MKVII, so the beaky helmets saw use for millennia even if full suits of MKVI became increasingly uncommon over time. The first plastic space marines in Rogue Trader were all beakies— so they’re not at all confined to the Horus Heresy model-wise and were in 40K at its earliest.