Requires at least two new game cycles and then killing Sif once more for weapons from boss souls. Remember to kill the optional bosses Gywndolin and Moonlight Butterfly on both playthroughs, and kill Super Smough on one and Super Ornstein on the other. The other repeat bosses are mandatory to complete the game.
The tail cutoffs for Priscilla, Seathe, Gaping Dragon, and Hellkite Drake are soft-missable until the next new game cycle as well. final soft-missables are lost on killing Nito. For Grant and Sanctus you allow Paladin Leeroy to invade you as a human as you approach Nito's boss fog and kill him for it. For the Gravelord Sword, become a Gravelord Servant.
The remaining weapons can be hunted down at any point before killing Gywn. Use the Gold Covetous Serpent Ring and 11 Soft Humanity to maximize drop rate of rare weapons you need to farm. Full list is here:
https://darksouls.fandom.com/wiki/Knight%27s_Honor