GetAllCompletedQuestIDs() but for world quest?
 

GetAllCompletedQuestIDs() returns all completed quests but apparently no world quests.

Is there really no way to find out if a world quest has ever been completed?

The answer is probably (and sadly) no, otherwise this addon would not have to manually count world quest completions.
But you never know... :)

I'm afraid no but happy to be wrong

I don't know of a way as they seem to be considered in the same realm as dailies. The loadable addon Grail-When just records when you did it (and the count) based on the quest being completed.

Cross referencing any related achievement(s) might work. I'm under the impression world quests all have achievements, I could be wrong, but most I've done have some kind of achievement for "completing" all the variations or simply repeating them multiple times; the criteria can be things like times the quest was completed, what rewards you've received, which NPC you performed the quest with, and others. Thus checking for the presence of a completed criteria would indicate the world quest itself had been completed at least once.

Also not sure on this but I believe all world quest related achievements contain the title of the quest, in double quotes in their descriptions, which should mean it's possible to scan the description of achievements for the quest title.