Query a database specific configuration parameter
These settings are stored in the pg_catalog.pg_db_role_setting system catalog.  You can search for the databases that have the above setting like

SELECT datname 
  FROM (SELECT datname, unnest(setconfig) AS setting
          FROM pg_db_role_setting 
          JOIN pg_database AS d ON setdatabase = d.oid) AS t 
 WHERE setting = 'default_transaction_read_only=false';