Can Yasuo Legend be used during the opponent's turn, and if so, in which cases?

Ruling: No, Yasuo Legend cannot be used during the opponent's turn. Everything in the game operates at "base" speed (meaning on your turn, with no chain and no showdown) unless it specifically states Action or Reaction speed.

Nuances:

• Only cards that explicitly specify Action or Reaction speed can be played outside of your normal turn timing