‘common knowledge’ essentially changes with every generation because life in general changes.
What we’ve lost in my lifetime is ‘basic knowledge. What I mean by that are the basics in science, math, history, critical thinking, critical listening etc.
Not too surprising. Everyone can access all human knowledge at light speed now.
Memorization was important when reference material was not at hand.
And to really stir the pot, even the "experts" can't agree on much of anything.
On the plus side, we now have more experts than ever before in human history!!