Just wander if there are any plans on implementing subject instead of polling http clients?
Not really - we’re more likely to implement HTTP 2 than web sockets.
There is also long polling support.
We have discussed doing a SSE interface but as @James said I prefer
http 2.0 as well
Indeed SSE is contraventional since IE didn’t implemented it, websockets in this regard looks much better.
As far as I know there is no HTML5 API to utilize server push in HTTP2, they are not like SSE and can not be used for realtime updates. HTTP2 push used to delivery static assets before they requested.
Here is quote from HTTP2 FAQ:
So I’m afraid your HTTP2 plan will not gonna work. And we’re back to SSE/Websockets 
BTW, is there a demand for non-polling HTTP clients from paid customers?
I’m asking because I’ve found an transparency report of yours, in this mailing list, about how features are planned and released, and paid customers’s demands were prioritized by 2/3 factor.
SSE is the most likely to be supported. There is a pending change I
need to make to the http api (if you look in the list you can find the
discussion about it (description document). Once that is in I don't
see SSE as being a huge project.
Cheers,
Greg