Placate reduces your opponent's war support by 10 and can be used once per turn. The ability to Placate is incredibly powerful because it can allow you to win wars without having to fight a battle.  Placate is especially effective against empires with several vassals because when you use Placate on a vassal, the war support is reduced from the liege. This means that if an empire has 2 vassals, you're able to reduce their war support by 30 per turn if you use Placate on them and each of their vassals. 

One strategy I've done is to make a bunch of demands against an empire with several vassals after I have high war support against them. I make all of my demands back to back and then I do an international crisis vote, which takes 4 turns to resolve, and during which neither side can make any additional demands. I use Placate on them and each of their vassals every turn as much as necessary to reduce their war support to 0 by the time they lose the international crisis vote. The obscenely high demands means that they will declare unsanctioned war rather than give in. You can then force them to surrender the following turn since their war support will be 0. 

You'll get everything you demanded and likely enough war score to vassalize them, making their vassals your vassals. I do this all game long to anyone who I have a lot of demands against. I've made over 90k gold doing this by having a dozen demands stacked up. The strategy can work without forcing the international crisis vote, which I only do to prevent them from making counter-demands and withdrawing them to gain war support. If they have no grievances against me then I don't need to use the international crisis vote and can likely pull off the strategy in only 2 turns.

Does anyone else do this or something similar?

i just declare war with tons of my demands and do same actions with placate. And after that leave them without teritories and having smt about 10k gold in medieval which can be named as win setup) But, in my opinioni think this mustn`t work in this way