NowTV Management have confirmed that Sky Sports Racing Channel will be available on the NowTV service from the 8th of January 2019.
Confirmation in this post here.
I came on to ask the same question.
I'm presuming they'll just add it to the existing 10 channels by 1st January 2019.
They say "available to every Sky customer at no extra cost." I won't hold my breath though. They'll find a way to charge NOW TV customers.