Geert:
From my observation and testing of quite a few image management programs, I've found that there are a very small number of programs that support "ALL" of the current IPTC metadata properties. Even Adobe Bridge seems to have stalled in adding support for new fields after the IPTC Extension updates of 2008.
However, it is important to note what Kirk said when he stated, "we will continue to preserve them if they're present."
Since the release of the IPTC Core — done in cooperation with Adobe — all new fields have been added as XMP properties. There have been a few rare exceptions, but most everything I have tested "preserves" existing XMP values, even if the application does not have a place in their data entry for a specific field!
Indeed, when I tested Photo Mechanic 6 recently (using the new Interoperability Test Suite
https://getpmd.iptc.org/interoptests-iptcpmd.html) I found a few fields that were not supported but were still persisted (which is better than finding them removed/deleted).
You can verify this for yourself (with PM or other applications) by taking a copy of the most recent IPTC Reference JPEG (located at:
https://www.iptc.org/std/photometadata/examples/IPTC-PhotometadataRef-Std2021.1-large.jpg), opening it in Photo Mechanic, changing a few values and clicking OK to write these changed values to the image. You can then submit that image file to Text #3 on the Interoperability Test page. You should mostly see cells filled with green squares. Cells with red in them indicate that a value is no longer present when compared to the original reference jpeg.
If you want more info on these tests check out the step-by-step instructions at:
https://iptc.org/standards/photo-metadata/interoperability-tests/Hope that helps.
David Riecks (for transparency, note that I am currently "Co-lead" of the IPTC Photo Metadata Working Group).