Google announced that additional enforcement is on the way:
Soon we will start to (1) disapprove products with incorrect GTIN, MPN and brand and (2) globally demote products that incorrectly use the identifier_exists attribute. We will continue to ramp up enforcements to find and disapprove products with incorrect GTIN, MPN and brand data in the coming months.
We recommend reviewing product warnings and disapprovals in the Data Quality and Data Feeds tabs in Merchant Center. For pending product disapproval launches, the violations may show up as warnings and eventually show as errors in the UI after the enforcement launch date. For products demoted due to incorrect use of ‘identifier_exists’ attribute, please review your products where it is set ‘false’ to make sure you marked so only for custom goods.