What are "old world restrictions"?

[Goregasmic]

So I went to the hole in hopes of killing the ghost of kindle and the only mobs in the area for pet purposes were A revenant

Looked them up and it says level 49-54, "The max level are unmezzable / uncharmable due to Old World restrictions."

Alright, I'll load up boltran and them not being 56+ means worst case I can stun and gate.

So I open with AE mez on a revenant and his skelly pet and I get a resist on the revenant. He's on me so I try to stun twice and both times the stun goes off, I get the blue emote "a revenant is dazzled by scintillating colors" but it does nothing, revenant is still wailing on me. Weird because usually when mobs are immune they return a resist message, on charm at least. Tried another mez and it got resisted too so I bolted before I got cooked.

Didn't feel like playing pet lottery with a paci timer running out so I ended up killing her with a different strategy.

It left me wondering, what are the old world restrictions/rules? I tried to google and it came up with nothing (and the AI answer was dogshit as usual).