A decoupled ceiling might eliminate most of the noise between floors, but not all of it.
If you're thinking of decoupling a ceiling in your home or apartment, you might not eliminate all the noise between two floors. This doesn't mean that it's not worth doing. It means that resolving noise issues in your home might be an iterative process. Because after you address one noise source, you might discover another one.
A noise path is how a noise enters another space. For example, this could be through the air or through a thin wall, through the joists and studs in a building frame, through a wall outlet, or through a vent. You might soundproof one noise source only to realize afterwards that there were other points of entry for that noise.
So if you're thinking about removing your existing ceiling and decoupling it with soundproofing materials, keep in mind that some types of noise might be coming from different sources via different paths. You might reduce or eliminate all the airborne noise between two floors, but not all the impact noise. It really depends on the type and construction of the flooring, the sills and the walls, among other factors.
This Soundproofist video outlines some of the noise paths to consider when you do soundproofing in your ceiling, and why you might still hear certain types of noise afterwards.