subtle seal
I think that most status effects should be capable of being applied to bosses, but of course there should be some caveats to that, as statuses like [Stun] could potentially trivialize bosses even more than some of the most extreme builds do. Statuses like [Bleed] though, should most definitely be applicable to bosses, as otherwise it actually hurts the viability of many builds that might incorporate or even depend on such mechanics.
I would recommend that statuses that are purely offensive, like [Burning] and [Bleed] should have full effect on bosses.
Statuses that could be consider "Soft CC" like [Chilled] or whatever else slows the target should be restricted down to a reduced effectiveness, so the slowed effect is only half effective than on normal enemies.
Statuses that could be considered "Hard CC" like [Stun], [Terrified] and [Frozen] should be further restricted and limited in effectiveness. Some ideas could include limiting these statuses to a (0.1sec) max, and the odds of applying these statuses are some value like 50% less likely. Additionally, such Statuses could be "resisted", and actually gives the boss an increasingly stronger resistance to such Statuses each time they are applied to the boss. So as the fight goes on, the worth of such statuses falls off, giving you a very small time table in which to rely on them.
rancid geyser