Oh, but at least it did.
Placate did hide you from from an individual enemy before the change. I was able to get full assassin strikes on enemies by using pacify on them before using an assassin strike.
I alt jump a lot, but I have several stalkers. I would quite frequently get to the middle of a fight, assassin strike a boss, fight with them until my assassin strike recharged, placate them, and then assassin strike them to finish them off.
But it was/is for a hide short period of time until it wears off. It simply did not grant you hidden status. It was simply a hide until the power's duration wore off.
Placate
Allows you to trick a foe to no longer attack you. A Successful Placate will also Hide you. This Hide is very brief, and offers no Defense bonus, but it will allow you to deliver a Critical Hit or an Assassin's Blade. However, if you attack a Placated Foe, he will be able to attack you back.
Now it is AoE. It was single target.
Pacify
Through persuasion or subtle intimidation, you attempt to placate a single target for a short time. If successful, the affected target will no longer attack you. This persists until the effect expires or you attack the target. Unlike other placation effects, this power will not hide you or give any concealment bonuses.
"unlike other placation effects" seems to be quite clear.