The NFL used to have a spearing call, which the Shazier hit would have been. I think it was done away with for the crown hit. A large part of the rule is focused on player safety - if you are hitting with the very top of your helmet (your eyes essentially looking down) vs your forehead, you can really F up your neck.
But yes, the Shazier hit should have been flagged.
I think Burfict's suspension is more than deserved, if anything it is too short. Burfict is a multiple repeat offender.
I don't know that Burfict would have been great in the 80's. Offenses had ways to get even, the most common being the crackback where a wide receiver would come back into the box and blindside the Linebacker or Defensive End.
In the NFL, they used to allow the crackback player to go low, ruining perfectly good knees.