Probably the biggest impact is in DPP NU, tbh, since there are numerous strong Dark-type Pokemon and few Dark resists in the tier.
Shiftry, Sneasel, and Crawdaunt all get way better as a result of having much better STAB to use.
For example, physical Shiftry used to be an easy switch in for Charizard (especially if no rocks, but even with rocks, it's doable if you have Roost). It doesn't mind Seed Bomb, Low Kick, or Sucker Punch on the switch in. It could explode, but that's at least a double down. Now, Charizard can't safely switch in on Knock Off, since it does >50%, killing if Rocks are up and if not it still neuters Charizard immensely the rest of the match by removing its LO or Lefties, etc.
SD Sneasel becomes more threatening now too cuz you can do things like this against relatively bulky neutral mons
+2 252 Atk Life Orb Sneasel Knock Off vs. 252 HP / 80 Def Muk: 381-448 (92 - 108.2%) -- 50% chance to OHKO
+2 252 Atk Life Orb Sneasel Knock Off vs. 252 HP / 0 Def Ampharos: 421-496 (109.6 - 129.1%) -- guaranteed OHKO
+2 252 Atk Life Orb Sneasel Knock Off vs. 252 HP / 4 Def Vileplume: 378-445 (106.7 - 125.7%) -- guaranteed OHKO
+2 252 Atk Life Orb Sneasel Knock Off vs. 252 HP / 0 Def Porygon2: 364-429 (97.3 - 114.7%) -- 81.3% chance to OHKO