There are two part to this.
- Is your iMac hardware capable of outputting 10bit colour aka. 30bit colour
- Does Apple themselves enable 10bit colour on your iMac in OS X
As a comparison I have a Mac Pro 2010 model with an AMD Radeon HD 7950 video card connected via HDMI to a Sony TV. AMD officially confirm this video card is 10bit capable and indeed if I boot in to Windows 10 via Boot Camp I do successfully get 10bit mode enabled and my Sony TV confirms this. Unfortunately all the same hardware right down the cable when used in OS X does not allow 10bit mode because Apple wilfully have crippled it. Apple do not even support audio on this video card even though again it is capable of it and it works immediately again in Windows via Boot Camp. 😟
Based on my experience you ideally need to boot in to Windows 10 via Boot Camp and see what happens, this will let you check whether your hardware supports 10bit colour mode, as an example a Nvidia GTX-680 card which can also be used in a Mac Pro 2010 does not support 10bit colour according to Nvidia and my own experience in both OS X and Windows 10.
There is a small but distinct possibility that Apple may have been unfriendly enough such that a new iMac can do 10bit mode on its built-in screen but might still not enable it for external screens.
Sadly Apple have proven very poor at supporting 'professional' features like 10bit colour. They also do not support H.265 video, or lossless audio standards like Dolby TrueHD even in their 'professional' software products like FinalCut Pro X.