subsampling automatically detected

I consider using jpeg-recompress at Fasterize. Fasterize automatically optimizes images from various websites.

So, images at the input may be uncompressed or already compressed at different levels.

1 - I wonder if there is a way to know if an image has already been compressed at a chroma subsampling 4:2:0 ?

2 - If my understanding is correct, jpeg-recompress picks the "right" quality score based on an image comparison. Is jpeg-recompress able to know which chroma subsampling is suitable for a given image ?
I think that if the input image is already with a subsampling 4:2:0, no more subsampling should be done.

The issue names jpeg-recompress but no files or tests. Start by locating its quality-selection and chroma-subsampling handling, then determine whether input 4:2:0 can be detected and preserved and whether selection can be image-dependent. Done means a documented answer or a concrete, tested behavior change.

Written by the indexing model from the issue text.