Create search keyword, analogous to [[ https://www.mediawiki.org/wiki/Help:CirrusSearch#Intitle_and_incategory | incategory: ]] but allowing to include subcategories in the search too.
Proposed name: deepcategory:.
This will be done by using SPARQL queries to WDQS categories data as backend: https://www.mediawiki.org/wiki/Wikidata_query_service/Categories