Debian's current appdata tests show warnings about missing metadata files. E.g. https://appstream.debian.org/sid/main/issues/freeciv-server.html
Apparently files formerly assumed to be named as *.appdata.xml should now be named as *.metainfo.xml ( https://freedesktop.org/software/appstream/docs/chap-Metadata.html#sect-Metadata-GenericComponent )
There's actually multiple levels of backward compatibility things here. Naming files as *.appdata.xml should be still ok ("Component metadata of type desktop-application ...can be installed with an .appdata.xml extension as well"), and I'm not sure we want to change that extension in the middle of a release series (i.e. in S3_0). But changing the directory where the files reside is more urgent (and the new location has been supported by the distributions for longer time, so should not cause any problems): "Support for the legacy path will likely be dropped completely with a future AppStream 1.0 release."
Updating the default directory for appdata/metainfo files: #44091
'make dist' failing (bootstrap/Makefile.am lists old file names)
- Only master patch lacked those bootstrap/Makefile.am changes. New version attached.