How could it be healthy to not even agree on the most basic facts of history like whether hundreds of years actually happened or the shape of the earth?
But they belong to a bigger trend which questions the value of experts and institutional knowledge. This could either send us into a dark age, or challenge a better class of experts and institutional knowledge. My money is on the latter. Few things improve without some kind of challenge.
It’s fine, and healthy, to disagree on how best to deal with the world as it exists, but if we can’t agree on reality itself, compromise and progress are so much more challenging.
But to help you understand, rewriting the past to sustain power is the type of thing that degenerates into genocide for example.
In the short term it might look innocuous, but if corrupting truth and suspend basic moral and ethic values on something as basic as History, what other basic human values are you willing to suspend because you don't like how something feels?
This is the breeding ground for racial supremacists, ethnophobia, etc. Terrible stuff.
And you don't need to go into the past to see the consequences, Ukraine is fighting for their existence because a group of people in Russia don't want to acknowledge that a ethnic group they deem inferior can have better chances at thriving than them, to the point of denying Ukrainians existence all together.