A Calm Togekiss with only 4 SpA EVs using Air Slash vs. a Bold Slowbro with 252 HP EVs and 0 SpD EVs has a 99.9% chance to 4HKO after Leftovers recovery,
according to the calculator. Air Slash has a 60% chance to flinch the opponent with Serene Grace. Let's not even involve Thunder Wave in this equation: let's simply ask what the odds are of flinching the enemy Slowbro every turn for four turns in a row. 0.6 * 0.6 * 0.6 * 0.6 or
12.96%. In other words, the odds of flinchhaxing a Slowbro to death without once allowing him to get an attack in,
even in the absence of Thunder Wave, is three times greater than the statistic you provided. By your logic of "since it can AND HAS happened," we must needs ban Air Slash + Serene Grace as well.
I should very,
very much like to think that we are not going to ban Air Slash + Serene Grace.