"Mainstream EXIF readers" are mostly derivatives/wrappers around Phil Harvey's ExifTool, which has supported IPTC tags for a very long time. Whatever may be interesting about this, the fact that the data is stored in an IPTC tag is not really part of it. IPTC/IIM structures predate Exif by decades: https://en.wikipedia.org/wiki/IPTC_Information_Interchange_M...