I am pretty sure that a statement for "Commons category" (P373) was sufficient to show a link to Commonscat in Wikipedia on the left sidebar under "In other projects". Since a few days only an entry in Wikidata as "Other projects" shows the link in Wikipedia.
Example (per today): The Commonscat statement in https://www.wikidata.org/wiki/Q48634 is not shown on https://de.wikipedia.org/wiki/Sarkophag