I see the "on hold" state on some questions that would have been closed in the past.
What is this (new?) state, why was it introduced?
Can I vote to close in that state?
There have been some changes to the close process. It's explained in this blog post and this post on the main meta site. And specifically, the "on hold" state is discussed at Every "close" has its thorn: replace "close" with "on hold" for the first five days.
Basically, on-hold is a temporary state for closed questions, meant to not feel as nasty and to encourage actual improvement.