The likely reason Crit Damage Reduction is being removed
Like Exhaust double dipping vs AS users, Randuins unfairly countered specifically those who build crit. Like Sivir, Trynd, Crit Jhin or Jinx. While doing nothing vs lethality users, AD caster or flat physical damage. Like Varus, Fiora, Vi, Lethality Jhin or Bloodrazor users.
Meaning an item already geared towards lowering AS champions was hitting a specific group harder than another, a group that is already worse than the other.
And no, Yasuo's annoying design and bloated kit don't justify such a thing. That is Yasuo's problem and should be fixed on Yasuo's end.
It is a positive change considering the overall improvement to tank items anyway, and the upcoming tank update. It will balance out balancing a lot more and make tweaking other items and champions easier.
In otherwords. It is aimed towards making future balancing more sensible.
But you know, circlejerks must circlejerk.