Basically, it’s as if Discovery never existed, which is exactly how it should be, according to what we see in Star Trek: The Original Series and all the shows and movies that followed until Discovery came along.
That would explain how someone like Captain Kirk didn’t know about Discovery, despite its integral role in the Starfleet and Klingon war.
This conveniently explained why Star Trek: Discovery’s high tech equipment is nowhere to be found in Star Trek, and why there are no mentions of the Discovery or Spock’s adopted sister Michael.