I’ve personally asked this question multiple times, and everyone always misunderstands and redirects the conversation towards the Staggered status when that’s not really what OP is talking about or why he’s asking:
Exploiter’s Aspect
You have 20% increased Crowd Control Duration. While enemies are Unstoppable, you deal [20 - 50%] increased damage to them.
This is what he’s talking about for everyone’s reference.
Now I’m still not 100% about this but from what I’ve gathered - no, bosses are not considered under the status “Unstoppable” when not Staggered and therefore this damage bonus wouldn’t apply constantly, even though they are immune to CC. There’s a specific broken-chains icon for the Unstoppable status (here’s a Barbarian’s Rally Cry skill w/ Unstoppable for reference) so I would assume any bosses that go into an Unstoppable status would also have this icon.