I hadn't heard of the term unmasking either but, from your description, it sounds like automatic eq.
Most people mixing a project are going to do that by ear, notching down certain frequencies or areas in, say, guitars or piano, to allow a lead instrument or vocal to be heard more clearly.
There's generally three tools you can use to move things out of each other's way.
Eq, panning, and distance (ambience/reverb).
For ducking I don't know how common it is to use it on vocals in music but certainly for dialog it's used a lot.
Any time you hear the music track disappear, on a radio show or podcast, right as the host speaks, that's ducking.
It's also very commonly used in electronic and beat driven music to completely clear the way for the kick.
It's often used to the extreme in that case - You could call it pumping.
I don't see why you couldn't use ducking on vocals in a mix but as with every 'fix'
it might be worth asking if there's a fix that can be employed earlier in the process.
If you're finding some instrument is drowning out your vocal maybe something could be changed in the recording process to eliminate that problem?