
Alright first off MAJOR SPOILERS for Godzilla: King of the Monsters so before you read this go and see it. *deep inhale* Ghidorah is not dead, now before you start saying "But we saw him die" allow me to elaborate, do you remember what was happening when he was finally killed, his head was moving around, and trying to regenerate, now do you remember what happened in Godzilla's second encounter with Ghidorah, one of his heads was removed and likely slinked off to regenerate. And begin the debate in the comments!
Look upon my works, you mighty, and despair

I don’t think he can regenerate from just the head. Like a salamander can grow a leg back but a whole salamander can’t grow from a leg.
“Banana oil.”- George Takei, Gigantis: The Fire Monster

CAN'T WAIT TO READ THIS!!!!!! JUST HAVE TO KEEP MY EYES CLOSED!!!!!!!!!!!!!!!!!!
If people weren't lazy, we wouldn't try to be efficient. If we weren't efficient, we'd never get anything done.

Nah - He's dead - I think (hope?) they are settign up Mecha King Ghidorah

Yeah, I don't think the body can regenerate from the head. Only limbs and heads can regenerate from the body, as long as his vital organs remain intact. We see Godzilla step on Ghidorah, crushing his body with immense force.
However, we do see Ghidorah's head still roaring when Godzilla has it in his mouth before exploding it with his breath. But I suspect the animation of his head would have been short lived anyways.

Makes sense, but I stand by my theory that we haven't seen the last of Ghidorah
Look upon my works, you mighty, and despair

Wait post credit scene?
Look upon my works, you mighty, and despair

There was a scene with it in the post credits. You had to stay to the very end though.

Yeah, I just saw it, the head was decaying, probably has something to do with the oxygen destroyer being used
Look upon my works, you mighty, and despair

'The body couldn't grow back from the head.' Not in the real world no, but heads don't grow back either. Tails, legs, sure but not heads. But anything's possible from this film

They do imply that the head will be used for something in the post credits scene. Perhaps man made monsters using DNA from the head? Remember, KG is an alien. We have no idea what it is truly capable of.