Yeah, this is usually discouraged because when someone incorporates something like this with their character it forces other players to acknowledge that your character is the canonical wielder of a weapon, who canonically did the feat to obtain it.
All other characters who are associated with said scenario are basically either forced to A) alter their backstory/character knowledge for your character to fit in with their story or B)…pretend you don’t exist.
For Sulthraze, you are basically saying you are the one who canonically cleared Zul’farrak. Any other character who is related to Zul’farrak in some capacity is either forced to acknowledge that you are the canonical adventurer who did this feat or to just ignore you…and what if there are other people with competing claims? It just winds up being a headache for everyone involved.
While I can’t tell you how to RP your character, you are of course free to do as you wish, but know that using a named weapon in lore - even if the lore is barely existent - is a fairly large taboo and will have an impact on people’s willingness to RP with you.
I personally would probably not acknowledge it simply for the bag of worms that it opens. If your character is canonically the one who cleared Zul’farrak and claimed this weapon, then why is that okay but other people’s claims that they were the ones who canonically cleared other dungeons/raids not okay? What if I encounter others related to Zul’farrak down the line, how do I resolve that?
Worse, what if down the road, this sword appears again in the lore? Not that its likely, but it wouldn’t be the first time a weapon that’s obtainable makes a reappearance in the lore. Examples include but aren’t limited to Atiesh, the Corrupted Ashbringer, hinted at Quel’Delar, how Ashkandi appeared in two different raids from two different ‘eras’, etc. etc.
With Zul’farrak, there is a canonical Cataclysm version and then the Classic version, would people who acquired the weapon in Classic have a ‘non-canon’ version of the weapon itself? What if a future revamp of the dungeon is made and the weapon is found there…does that mean that this hypothetical future version is non-canon? Or would your character have just magically lost and re-acquired this sword?
Its far easier for me to ICly ignore it than to engage. Not saying that your RP is bad, but I don’t want to be involved in deciding who canonically did what and all that nonsense.