KDBX4 has CustomData (String dictionary) fields for PwDatabase, PwGroup and PwEntry. "PwEntry and PwGroup now have a CustomData property. This property has the same type as CustomData of PwDatabase, and is serialized analogously (same XML structure)." (https://keepass.info/help/kb/kdbx_4.html#extobj)
KDBX4 has CustomData (String dictionary) fields for PwDatabase, PwGroup and PwEntry. "PwEntry and PwGroup now have a CustomData property. This property has the same type as CustomData of PwDatabase, and is serialized analogously (same XML structure)." (https://keepass.info/help/kb/kdbx_4.html#extobj)
This would enable nice features for projects using your library. https://gitlab.gnome.org/World/secrets/-/issues/445