According to Reddit, and Reddit always has the most accurate information (/s just in case): Shield TV indeed can't do 4:4:4 12bit in 4k 60fps due to the limitation of HDMI 2.0b. But it could do 4:4:4 12bit 4k at 24fps or at least should be possible.
	
		https://www.reddit.com/r/ShieldAndroidTV/comments/ktu8zt/_/giogm8m
Here's spec sheet for shield tv pro:
	
	
		
			
				
			
			
				
				Buy NVIDIA SHIELD TV PRO with AI-enhanced upscaling feature in the market today
				
					
						
					
					www.nvidia.com
				
			 
		 
	 
It mentions these points:
Dolby Vision HDR and HDR10 
Up to 
4K HDR playback at 60 FPS (H.265/HEVC)
Audio support: AAC, AAC+, eAAC+, MP3, WAVE, AMR, OGG Vorbis, FLAC, PCM, WMA, WMA-Pro, WMA-Lossless, Dolby Digital Plus, 
Dolby Atmos, 
Dolby TrueHD (pass-through), DTS-X (pass-through), and DTS-HD (pass-through