So far there hasn’t really been much rumors regarding the D5s, but recently a hashtag spotted on Instagram for the “#nikond5s” has fueled speculation that the camera is currently being tested in the field. The hashtag was spotted on the photos of Nikon Ambassador Ami Vitale which were shared on Instagram.
Sure, it could be a typo, but as Nikon Rumors points out, Vitale’s previous posts have been pretty accurate when it came to hashtagging, so it seems unlikely that she would have a typo not just on one photo, but on several. Other users in the comments have also pointed out that Vitale is one of the more highly regarded ambassadors of Nikon, meaning that if Nikon were looking for a photographer to field test the D5s, she would definitely be one of them.
In any case it is all just speculation at this point in time as Nikon has yet to officially confirm anything. However CES 2018 is around the corner so it could be announced then, or alternatively CP+ is another potential platform for Nikon to make the official announcement.