Remove childElementCount as it is redundant with children.length. If this is done I can remove it from DOM Core which would be nice.
It is quite strange to remove something which was implemented and specified (even Rec level) just couple of years ago. I don't recall why Element Traversal didn't include .children
It seems better than removing it a decade from now as we try to do with other badly designed features.
I'd be ok with attempting to remove it. And I definitely agree, it's better to remove something early if it's a bad feature (which I think is the case here), rather than wait and try to remove it later.
This page does not say anymore that this is going to be removed. https://developer.mozilla.org/en-US/docs/DOM/Element.childElementCount