Statistics on them are; we can tell you about numbers. Individual warning letters are not, unless someone goes through the access to information process.
There's a privacy right involved there: the person who was subject to the warning letter has a privacy right. There's no finding of guilt with a warning; there's no admission of guilt; and there's no particular standard of proof, if an officer has noticed something and is bringing it to the attention of a violator. It is still subject to privacy laws.