Paul says in 2 Cor. 5:16 that "even though we once knew Christ by means of the flesh [ kata sarka], we know him thus no longer What was Paul trying to say here? 
"We know him  thus no longer"
The question seems to be finding something that simply isn't there.

Consider other translations of this verse, e.g. CSB:


  From now on, then, we do not know anyone from a worldly perspective.
  Even if we have known Christ from a worldly perspective, yet now we no longer know him in this way.


It's talking about how we should relate to other people, at a spiritual rather than physical level.

There's not the slightest insinuation that any of Jesus's teachings are no longer relevant.