The creator has an incentive to pay attention to the deadline and provide correct data. If the `deadline` is set in the past, the campaign will be completed. If there are any funds the creator or the contributors (depending on the success of the campaign) can receive them. It is the creator's responsibility to set correct deadline, otherwise the creator can create a new campaign. There is no impact on the protocol from this missing check, so I consider this to be an informational issue.