This is an odd bug I just noticed today. The 2H sword, Typhoon, has vanished from the game! 
Have any other transmog appearances mysteriously vanished?
I wonder if it has something to do with them messing with ID’s for this anniversary event and the corresponding loot? This could mean that a lot of other really old world boss items may have vanished from peoples collections.
Hey there Raijin.
I would encourage you first of all to check your filters to make sure you haven’t accidentally filtered out the sword.
Keep in mind as well that the character needs to be able to use the item for it to show. So your Mage for example wouldn’t see the two-hand sword in their list. 
We have a handy overview on our support page that outlines the requirements and limitations.
If you find that the character who had the sword is otherwise eligible in all ways I would encourage you to open up a ticket so that our Game Masters can investigate further what happened to the Sword. And please also make sure to report it as a bug.
The more information we can provide to our developers the easier it will be for them to prioritize what issues to investigate further and hopefully identify potential issues.
I’ve done all this. It still doesn’t explain why the item no longer exists in the appearances tab (i.e. greyed out if not owned). Make a Warrior toon, Collections window, Appearances tab, Two handed swords, Search Typhoon.
I looted it during the last anniversary on my warrior yet in-game support tells me I’ve never looted it. It’s just a transmog item so it’s not game breaking, just incredibly annoying.
Edit:
Farmed it again on my plate toons and dropped it. Will put it in storage this time and not trust the appearance system to hold it 