The MB Air is the closest we'll get. It's basically a 'luxury' netbook.
I always thought the MBA was a kind of high-end Netbook.
I don't think it's fair to call the MBA a high-end Netbook for one specific reason. One of the most common aspects of all Netbooks is that they are small and light. The MBA is small, at least in the sense that it's so thin that it's very easy to fit in with your things, but it's hard to call it a "luxury" netbook when it outweighs almost every other netbook, sometimes by a factor of more than 50%.
The other core aspect of a Netbook is that it is designed for lighter computing -- while they all run full desktop/laptop operating systems, the emphasis is usually on rapid-start access to the web, e-mail, etc.
There, I can see that the MBA has many aspects of a netbook, aside obviously from the idea that these devices were purposely made with just enough power to accomplish these tasks to keep costs down, which obviously doesn't apply to the MBA.
OP: There is a more detailed rumor of fairly good credibility that Apple
does intend to release another sub-notebook.
https://forums.macrumors.com/threads/538125/
I didn't merge threads because it really isn't clear that this device will be built on the idea of a netbook any more than the MBA is.
EDIT: There is also this very odd rumor...
https://forums.macrumors.com/threads/573998/
But it's really not clear
what it's about or whether it will even turn out to be real or just a hoax. Supposedly Apple is developing a new device of some sort under the codename Brick that is different from a new laptop in some sense, but also related. Meh.
