The real problem is that removing any concept is the same as removing any emotion from a human. The idea of removing "fear" from a person sounds great - they'll be brave from now on. Until they start sticking their hands in a fire because they aren't concerned about the consequences. Think about what car travel / pedestrians would be like if nobody had the concept of fear?
Personally, I think removing something as big as the concept of
truth from the minds of all humans would have a similar-level effect to that. "Truth" is the sense of that something can be known, e.g. that it can be predicted, and since it can be predicted, you can fall back on heuristics to quickly determine what's compatible with that. With no sense of truth, humans would become a race of neurotics. e.g. people would wake up and lie in bed because they can't be sure life outside bed exists. They wouldn't put shoes on because shoes could just be an illusion. You'd have to have that debate every morning. Because when you take away "absolute truth" it is taken away for the big questions, but also the small question that we normally don't ask.
So you
definitely don't want to mess with things by removing a core part of human psychology that people use to navigate the world unless you want a disaster world where nobody can function at a basic level of normality. You want to remove one more or less useless concept which also causes problems. "Tribalism" which underlies patriotism/nationalism/bigotry would be my choice. However, then the consequences would be that groups aren't cohesive, e.g. things like special interest clubs wouldn't be stable since members wouldn't feel the need to associate with other members more than with a random person they don't know. Still less disruptive than removing "truth" but with most of the benefits.
The notion of absolute truth. People would immediately understand the scientific method, and we'd quickly find ways to deal with all the PoMo weirdness. Also folks' communication skills would improve since they'd no longer assume the other person thinks the same way they do.
Well you can say that, but just removing the concept of truth doesn't mean people would understand jack shit, sorry to say. They might as easily turn to scrying magical chicken guts or something. Then if the reading is disliked, read more chicken guts until you get one you like. Then you turn around and say "go for the scientific method it's better than chicken guts!" and they go "oh really? well that's just one theory".