AV1 supports YCoCg, which encodes RGB losslessly.

It is a bit-reversible rotation of the RGB cube. It makes the channels look more like luma and chroma that the codec expects.

False.

8-bit YCoCg (even when using the reversible YCoCg-R [1] scheme) cannot represent 8-bit RGB losslessly. The chroma channels would need 9 bits of precision to losslessly recover the original 8-bit RGB values.

[1] https://www.microsoft.com/en-us/research/wp-content/uploads/...

AVIF supports 10 and 12 bit encoding, which losslessly fits the 9-bit rotation of 8-bit data.

It's also possible to directly encode RGB (channels ordered as GBR) when you set identity matrix coefficients, it's just less efficient.

I've implemented this in my AVIF encoder, so I know what I'm saying.

Show me any of the popular image conversion tools (avifenc, imagemagick, photoshop, ffmpeg, whatever...) that does the identity matrix hack when asking for lossless AVIF. None of them do it. Many people have been burned by "lossless" AVIF, where they converted their images in the mistaken belief that the result will be bit-identical to the original, only to find out that this wasn't the case, after they've deleted the original files.

That's shifting the goalposts from what the standard supports to the current state of the ecosystem. It's certainly an interesting point though. If common implementations all have bugs regarding lossless encoding that's a pretty bad situation.