I was made aware that GExiv2 contains a file with an unknown license and provenience.
Since it is part of the rather awful callback-based stream API, I will be removing the file and the corresponding API ASAP.
A quick search around the internet looked like that currently no one is using this API.
I also have a prototype for reading and writing from GBytes that should cover some of the use-cases of that API.
Additionally some 0.27.x version in exiv2 seems to have fixed the issue that prevented the from_stream api to work properly, so I will undeprecate the gexiv2_metadata_from_strream()