2021.08.26 17:59 "Re: [Tiff] Setting/Getting Custom Metadata", by Even Rouault
I've some vague recollection of having looked for what would be the "new" interface and failed in doing so (my impression was that such a new interface was never completed). In the software I maintain, I use TIFFMergeFieldInfo and that works fine for ASCII tags. See https://github.com/OSGeo/gdal/blob/028757368b9064575b133b2bd9e6a928b69915cb/gdal/frmts/gtiff/geotiff.cpp#L19779
My software is free, but my time generally not.