Surely the point is, MOST people WON'T have cold iron on them at all, and also, many would not even know about this weakness link with Fey creatures and beings, not least as they're unlikely to meet them, and anyway, in most cases, Fey will never be encountered except in the rarest circumstances and when they are, their superior senses and innate magic tends to sniff out mortal beings long before said beings even KNOW there's something watching or stalking them, never mind what it is! By that time the Cold Rider/Nuckelavee/Satyr/Dryad/Korred/Shadow Collector etc. has already probably done its worth. You make it sound like cold iron's so common that this in itself will make all Fey extinct! What rubbish. Fey are virtually immortal and all are tough and for every Fey that dies by whatever cause, surely a 1'000 mortals will fall all over the globe. Fighting them again is also a toss-up. How do you know you'll win. A few successful strikes on a formidable Fey adversary will not fell it before it likely fells you. That's always supposing you've survived intact the many spells it's likely thrown at you first even before you reach it!
Some fey sound easier than others. Dryads, for instance, are tied to specific trees. Chop 'em down and they will die. Also, bludgeoning instruments are deadly to them whereas slashing and piercing virtually do nothing. But you can make things easier. Fairies and all Fey are not able to see non-fey folk if they wear red or their clothes inside out (how that works with armour, one never knows, unless you colour the metal red. Running water is said to disrupt their magic from working successfully (so all bodies of water in Faerun are still?) and certain magic items like a River Stone-a circular or oval stone with an iron ring at its centre ensures no Fey-even Hag or worse can come near you!
Poison is a good idea as a back-up, though I doubt on more Undead-linked Fey (Cold Riders/Nuckelavee/Escorite/Grimstalker) they'd work at all. Fire and "magical-weapons"-without it going into just WHAT the exact enchantment is specifically IN or ON the weapon should work too. But I did hear than any Fey killed away from the Feywild-i.e the Prime World simply reform from scratch in the Feywild, in a sketchy prose now hard to find. Presumably this makes them very Fiend-like, but that does that also not mean that if killed on their own turf, they're dead forever?
You could still note that Cold Iron weapons have the same effect (but it's unlikely anyone has those lying around.
