$wgContentNamespaces should include the "Thésaurus:" namespace on fr.wiktionary.org.
On of the consequence of not having it included is that it's not mirrored in ZIM files.
The consensus is on https://fr.wiktionary.org/wiki/Wiktionnaire:Wikid%C3%A9mie/avril_2015#Ajout_d'une_namespace_%22Th%C3%A9saurus%22_dans_la_liste_des_namespaces_%22de_contenu%22