Adding or removing keywords via -keywords+=... or -keywords-=... does not seem to work with HEIC files. Exiftool reports the files as unchanged.
It works for JPG files without a problem.
If you take a look at the Supported File Types table (https://exiftool.org/#supported), you'll see that HEIC does not support IPCT IIM/Legacy.
Use Subject instead.
For HEIC, keywords is in XMP. However, only setting works - adding or removing does not. The same applies to XMP files.
Keywords in XMP is XMP-pdf:Keywords. If you take a look at the table for XMP-pdf (https://exiftool.org/TagNames/XMP.html#pdf), you'll see that the tag is a simple string, not a list type tag. So you can set it or remove it as you would other string tags such as Description, but you can't alter it with list type tag commands.
Understood. Thanks for the quick replies!