This item gives a permanent -1 bag slot debuff and there is no way to cleanse it. Please fix.
Edit: 5 Month Update.
As Kazecap linked below, according to the support article, Blizzard’s official stance on this so far seems to be that they designed this item to permanently reside in our inventories.
If you’re reading this, please mark the support article as unhelpful. Maybe if enough do so a dev will actually look at it.
It makes no sense at all for the item to be designed this way.
Please let us delete this item! I’ve gotten Slick, I’m done, please let me get rid of it…a permanent item stuck in my inventory is no fun.
Can confirm, this item can’t be deleted even after obtaining the permanent version of the mount, Big Slick in the City.
There is no reason for this item to be undeletable, you could simply allow it to be deleted but re-obtained at any time from the Glimmerog Racer NPC.
Chiming in to add a +1 to this request. Received the full mount version of Big Slick today and was surprised in a bad way to discover that we can’t delete the Conch Whistle item. Absolutely don’t understand why the quest item cannot be reattained from the Glimmerog Racer NPC, as mentioned by others.
If I had known this little bastard would live forever rent free in my bank, I would have never petted it.
Still unable to delete the Conch Whistle from my bags - May 30th… Please fix this… My OCD is triggering
Still unable to delete the Conch Whistle from my bags - June 15th 2023… Please fix this…
still cant
While we’re on the topic, I hate items that need to be deleted from your bags. It ruins the immersion of the game. However in this case since it wasn’t implemented prior to introduction to live servers, deleting is probably the best option.
As others have said, if you allow it to be re-obtained from the NPC, it should resolve any issues. Maybe also have it auto-removed when the final permanent mount is obtained.
When the quest is completed, we should be able to delete the item. I have read the article that claims this is intended, with the explanation “needed for multiple quests within the game”. This is unacceptable. You have never forced undeletable items into our bags in the past.
Please fix for 10.1.7 
For the love of all that is good, Blizzard, LET US DELETE THIS DAMN CONCH WHISTLE.
I don’t want it in my bag!
Same issue as everyone else, can’t delete the Conch Whistle.
As of patch 1.5, STILL can NOT delete this…
STILLLLL cannot delete the Conch Whistle… July 15th >:(
Bruh.
Just let us delete it and pick it back from the NPC at a later date…
What a joke lol. I sure hope some new AI tool they’re using wrote that solution. Surely anyone with a brain wouldn’t purposefully make a new quest in 2023 that permanently removes 1 bag slot as a reward.
The Caverns is almost outdated. It isn’t needed for any normal questing anymore. No different than a deleted hearthstone - let it be reacquired from person X. And it’s only needed for a particular subset of quests, that nobody will do once ‘completed’. This doesn’t make sense otherwise to hold on to. More so when we’re all struggling with bag/bank space and creating alt guilds/chars to store more.
That support article is just hilarious. It was needed for like one intro quest and two dailies, and you can’t even see the dailies anymore once you have max rep and the mount.
And yeah, like you said they could have just made a dialogue option that gives the quest item back. It’s like some junior dev designed that whole thing without any reference to past precedent.
August, still can’t delete, done with all quests, have mount…
september and this dumb thing is still in my bag…