I tried to make the outdated metadata-exiv2 branch current in July 2022. It is that attempt that is recorded in the most recent posts here: CR3 metadata decoding support · Issue #6248 · Beep6581/RawTherapee · GitHub in the issue report pointed out by @Thanatomanic.
No PR was ever created, because there is an issue with the original code in the metadata-exiv2 branch. The issue is, as I see it, that even if CR3 metadata is visible using metadata-exiv2 branch (this works!), or a current version of it, the output from RT does not contain any embedded color profile at all. This issue was also confirmed by other tests done by @jhmnieuwenhuis.
The discussion is here: CR3 metadata decoding test of feature branch on Linux.
It is correct as stated by @paperdigits that metadata-exiv2 branch requires exiv2 v0.27.5 from GitHub - Exiv2/exiv2: Image metadata library and tools built using -DEXIV2_ENABLE_BMFF=On, to work.
I was able to resolve merge conflicts in outdated metadata-exiv2 branch. That was not hard, but I was not able to fix the issue of metadata-exiv2 branch code producing RT output files (TIFF, JPG) without any color profile embedded.
I am not in a position to give strong advice how to best proceed:
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Make metadata-exiv2 branch current again + fix missing embedded color profile in output
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Start from scratch again.