/usr/share/icon-naming-utils/legacy-icon-mapping.dtd is in icon-naming-utils 0.8.90-2.
This file is owned by root:root, with mode 0o644.
The actual contents of the file can be viewed below.
1 2 3 4 5 6 7 8 9 10 | <!-- The global elements of the legacy icon mapping XML -->
<!ELEMENT mapping (context+)>
<!ELEMENT context (icon+)>
<!ATTLIST context dir CDATA #REQUIRED>
<!ELEMENT icon (link+)>
<!ATTLIST icon name CDATA #REQUIRED>
<!ELEMENT link (#PCDATA)>
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