Digital Negative or DNG is an image formats developed, patented and licensed by Adobe. It is based on open standards and has a freely-available specification and source-code-based SDK.

It is compatible with TIFF/EP (an ISO standard format for ‘electronic photography’) and many open standards like Exif metadata, XMP metadata, IPTC metadata, CIE XYZ coordinates (colour) and JPEG (for previews).

Does source-code-based SDK mean source-available?

JPEG XL supports EXIF metadata and XMP metadata according to Phil Harvey, ExifTool author.[1]

See metadata.