We have been using NVENC with 444 and h264 in production going back years and HW as old as Turing.

But it turns out you are right, 444 decoding with H264 doesn't seem to be supported up until recently:

https://developer.nvidia.com/video-encode-decode-support-mat...

Never noticed, seems like a strange omission, but in any case doesn't seem to apply to newer codecs like AV1.