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