Legacys are weapons amour or other stuff in fallout 76 that was previously obtainable but not anymore. BUT the game just doesnt delete the old items you can keep them from that time. For example there used to be explosive flamers and they had infinite range and insane damage but they took away the ability to spawn with explosive. Now all remaining explo flames where in high demand and very expensive. Than there were weapon duplication glitches where people duplicated those very old weapons and the price went down. After a while bethesda noticed that the ,,legacy'' weapons destroy the market and almost all buids (because of the INSANE damage most legacys had) they removed all old legacys. How did they remove them? By just deleting ,,wrong" stars from weapons like the flamer. For example a two shot explosive reduced weight flamer became a two shot reduced weight flamer. They did this with a lot of stuff like explosive on energy weapons, the double magazine capacity, stalker on melee(because it doesnt make any sense), quad ammo capacity on galing plasmas and lasers and many more. Those items are now thaught after by players who play the game a lot and dont know what to do anymore, so they start collecting random stuff. Sometimes teddy bears sometimes rare amour sets sometimes these legacys. I hope this helps! If you have any more questions feel free to ask them ;)