I believe,
though I can't find an unambiguous reference,
that a tool or user is allowed to define and populate any XMP/Exif attribute it wants,
within the technical limits of attribute/value definition of the relevant metadata.
Whether or not anything else can read and make sense of that metadata is a different problem.
draw.io probably . It's pretty good for a free tool. If you lost the xml I think it even work to import directly an exported picture (I guess they append the data into it too).