Mistress of Pain has lifesteal because lifesteal is a evergreen keyword. Echo is not a evergreen keyword, so Witches Brew doesn't have Echo. How hard it is understand ? using keywords just for the sake for it is bad.
Do you mean it is a matter of consistency between expansions? Ok, then why don't they change EVERY FITTING card text when a new keyword appears, and, when an expansion is no longer in play, change EVERY FITTING card text BACK? That would be 100% consistency! Why? Because it applies to both standard, arena and wild modes.
But nevermind, i'm asking for consistency in a game that doesn't allow me to skip the text bubbles in the single player content and, as well, isn't capable of showing all the rewards or card changes with one click (after starting the game) instead of clicking through all of them like a stupid click simulator. I won't even care to discuss about a feature to discover all cards from a pack with one click... So much forced hype and idiocy.