South-west to Novigrad in the waters you will find the wreck of the “Flying Steg” ship. Read the journal located inside for an update to the next location. Just enter, go down the stairs and find the shiny chest where the Legendary Feline steel sword diagram is hidden. There are no enemies around (except for a couple of lvl 59 bandits, perhaps). The first diagram is in the basement of the Drahim Castle Ruins in Central Velen. If you need more information, check out the video. Here is a detailed description on how to find the diagrams. Best is really to just go to the diagrams locations and pick them up yourself. Can’t say for sure if CDPR removed it from the vendor or what happened. When I checked it prior to starting the hunt myself, I only found the Upgrade diagrams map there, not the original one. The map to start it was available for a cheap price on the Blacksmith vendor in Blackough. All you have to do is follow the guide to one of them, pick it up and the quest will appear in your log. The quest can be started by simply obtaining one of the diagrams.