Adding to this because I think it’s important in case anyone else runs into this problem.
It is true that the UI by default hides quests that are far below your level. The problem arises when this behavior changes.
Ex: Quests that offered full XP at level 49 (post SL squish) and we’re still visible at 50 and offering full XP, became marked as Trivial upon hitting level 51 (the XP it offered was 850 out of 21500 for a full level, so not insignificant).
This became problematic because 1) it goes against the common believe that trivial quests are significantly below your level and/or worth no experience and 2) upon ticket submission, there was no suggestion or mention of this being the behavior by the Game Masters that were assigned to my ticket.
Instead, I received the typical suggestions of clearing the interface, cache, and WTF folders. I even went so far as to reinstall the game. Upon finally being able to identify the quest giver and complete the quest, oddly the next quest in the chain was visible to myself. So I thought, designers be damned, it was truly a glitch and I managed to get through it.
However, the problem came back again (I was working on the main quests needed toward unlocking the Zandalari trolls, if that gives anyone reference, and my first problem started with Fuel for the Voodoo which was visible on my map, but not the minimap or with visual LOS of the questgiver.)
It was only after this that I pleaded with General chat and very quickly got a response to try showing Trivial quests.
Personally, I believe this is something that should be fixed. I understand the push for people to level to the cap in current content, but I think there are better ways to achieve this. With the level squish, the threshold of significantly below your level has obviously changed to something relatively insignificant. Most people would ideally rush into new content anyway.
TLDR; before you start resetting files and assuming it’s your addons fault, show your trivial quests.