A developer created a landing page in CloudPages which return unique content when subscriber data...

To control the rendering of unique and default content based on the presence of subscriber data in a related data extension, the best practice is to use the RowCount function and an IF statement:

%%[ VAR @rowCount SET @rowCount = RowCount(LookupRows("RelatedDataExtension", "SubscriberKey", _subscriberKey)) IF @rowCount > 0 THEN /* Render unique content */ ELSE /* Render default content */ ENDIF ]%%

This approach checks if there are any rows in the related data extension for the subscriber and conditionally renders the appropriate content.