As proposed here, Wikidata should have a [citation needed] button allowing users to request a reference for unsourced statements, so we can study and prioritize where citations are most needed.
I imagine this could work like a one-click "flag/star" button in the UI (with a counter) next to each statement.
Ideally, clicking that button would translate into something SPARQLable attached to that statement, e.g. a reference of the kind "within focus list" (P5008) "WikiProject Citation needed". See also citation needed constraint (Q54554025), which works on basically any statement involving a property, rather than a specific statement.
This was also suggested by Andy Mabbett over 2 years ago "We should instead devise an equivalent of Wikipedia's 'Citation needed' tag. " - https://www.wikidata.org/wiki/Wikidata:Requests_for_comment/Verifiability_and_living_persons