Debian Lintian reports the following error:

E: denemo-data: appstream-metadata-legacy-format usr/share/appdata/org.denemo.Denemo.appdata.xml
N:
N:   The AppStream metadata contains the obsolete root node <application>. It
N:   was used in a a legacy format. The application metadata for your package
N:   should follow the new format described on freedesktop.org.
N:
N:   You can validate draft formats with 'appstreamcli validate'.
N:
N:   Please refer to https://wiki.debian.org/AppStream/Guidelines and
N:   https://www.freedesktop.org/software/appstream/docs/chap-Metadata.html#sect-Metadata-GenericComponent
N:   for details.
N:
N:   Visibility: error
N:   Show-Always: no
N:   Check: appstream-metadata

and the following warning:

W: denemo-data: appstream-metadata-in-legacy-location usr/share/appdata/org.denemo.Denemo.appdata.xml
N:
N:   AppStream metadata file was found in /usr/share/appdata/. The AppStream
N:   XML files should be placed in /usr/share/metainfo/.
N:
N:   Please refer to https://wiki.debian.org/AppStream/Guidelines for details.
N:
N:   Visibility: warning
N:   Show-Always: no
N:   Check: appstream-metadata

The warning regarding the legacy location may be fixed with this command:

sed -i "s/appdata/metainfo/g" $(git grep -l appdata)

But modernizing org.denemo.Denemo.metainfo.xml would take more work.