Hmm, good question.
For reference, IPTC keywords:
http://www.iptc.org/std/photometadata/specification/IPTC-PhotoMetadata#keywords
I gave GIMP-2.10.4 a test. There is a “Keywords” section, but nowhere does it say where it’s stored. Testing shows they’re stored as XMP. Metadata handling was buggy. Time to write some bug reports:
https://gitlab.gnome.org/GNOME/gimp/issues