[x4e]

Darktable supports it under JPEG2000: https://docs.darktable.org/usermanual/4.0/en/overview/suppor... (Darktable also supports JPEG XL)

[chungy]

JPEG2000 is a different format, just as JPEG XL isn't the same as original JPEG.

[JyrkiAlakuijala]

12 bit jpeg is a different format in practice. Better to move forward than adding support for a new ancient format.

Also, jpegli can support 10+ bits within the old compatible "8 bit" formalism.