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

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

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.