> when converting from jpg to webp, XMP and exif metadata is lost.
https://crbug.com/webp/166 deals with an imbalance between the library based (libpng, libjpeg, etc) and native windows importer (WIC = Windows Image Component). You can try the -nowic binaries [1] as a workaround.
For the question in the subject we never defined an IPTC chunk for the format [2] -- I don't remember if there was a reason or it was just an oversight --, but it could be added to a file as the chunks are extensible. This would require some custom file manipulation currently as webpmux doesn't support adding arbitrary chunks to a file [3].
IPTC was not really widespread at the time the chunks were defined. Moreover, it's been recommended[1] since to embed IPTC data into XMP instead of using the separate IPTC metadata format, which is loosely supported.
I do not consider myself competent enough to write a custom RIFF container for IPTC metadata in a webp file. I have decided to put my IPTC metadata within XMP tags for the moment. maybe one day IPTC will be standardised for webp.