This was not a thing in vanilla and its terrible. Just destroyed a 30min duration quest item to make some bag space and it deleted the quest as well (was too fast on the trigger).
It will also remove quest items you put on the bank on quest deleteion.
Sometimes you need to make some quest log space for dungeon quests and it was a convenient way to just put them on the bank and delete the quest.
This is a good example of QoL changes gone wrong.
I can manage this on my own - thank you.
I donāt feel like this is the worst change, but I feel you, I think perhaps there should be a prompt āwould you like to delete items related to this quest as well?ā
the worst is it will even do it when you try to delete an extra one, say if you somehow got 11/10 quest items and want to delete that random 1 taking up bag space
Actually, that never happened to me.
I was suprised that system is that smart. Tried it multiple times (Deepmoss Eggs etc.).
Classic should be as it is/or/of/if/meme/etc. what game is based on.
Think Electric ā¦
Keep your feet on the ground/wire/switch/pathā¦
Good example of this, was in LBRS doing a quest run on my mage, the quest where you have to loot the 15 spider eggs, and looting a spider egg may cause to spawn a bunch of 5-6 adds, which would be great to come back there after the dungeons over and try to keep opening eggs to proc these little spiders to farm for xp, but once you get to 15/15 you canāt open any more eggs and if you were to delete some inorder to open them, you get prompted with the quest deletion 
Iām pretty sure you could delete quest items in Vanilla without losing the quest. That āfeatureā got added in much later. I kind of think it came in with the Cataclysm revamp but Iām not positive.
Yet Iām still cleaning my bagspace of quest-items that are hanging around long after the quest is finished, lol.
It is very inconsistent.
Itās like they took a QoL change from a newer expansion and put it through a meatgrinder (like a lot of other stuff too).
One could title this as āhalf-a*sedā.
So I recently deleted a quest item, but my quest stayed. Either this only happens on certain quests, or certain type of quests, or itās random and a bug.