Since 1.11 Creepers that have a Potion effect will spawn a new AreaEffectCloud (Lingering Potion cloud) with the effect of the Potion. There is currently no event being called when the new cloud entity is spawned and therefor no way to disable such behaviour.

Calling the LingeringPotionSplashEvent there would make it compatible with existing solutions that listen on AreaEffectCloud creations. An alternative would be to maybe add a new event that extends the EntitySpawnEvent. (as this is not called by the clouds currently)