That's because there isn't an option to untrack a quest. Never has, actually. There's a mod that provides in untracking quests, but why the vanilla game never had one, is unknown. Although, I remember vaguely to have untracked a quest once by accident, but have no idea how or why that happened.
This was maddening earlier on when the game was released and further. One that I really wanted to unfollow was Epistrophy because that quest was entirely annoying with Delamain calling you every two seconds or crashing into you while you were already doing another quest or whatever. To this day, I still haven't done that mission because I don't want all those cab listing in my journal. I do mean to try it again though as I understand that it has been vastly improved.
Other than that though, I haven't felt like I needed such a function.
What really drives me insane is that the game overrides what quest you're following whenever you get an update on a more important quest so to speak. So even if I manually set a quest to follow, the game still goes "nooope we're changing it for you!". I think this was a thing in the Witcher 3 as well if I remember correctly and it's sooo annoying.
If I manually follow a quest it's because I want to follow that damn quest.