As SoHo’s iron-boned, sprawling lofts became gold mines over the past two decades, co-op boards, banks, brokers and the city itself winked at a rule requiring that they be reserved for working artists.

But over the last year or so, something odd began to occur: people started paying attention to the rule.

Apartments, even those in buildings with the prestige of famous residents, have languished on the market. Banks began withholding mortgages. Co-op boards began ordering residents to apply to the city for certification as artists.