There are no clear cut standard of rules to follow in Journalism, journalists in general follow an unwritten code of conduct of how to report things or how to write a story and in general it creates a sort of method that they all follow, I don't think there is any reason to have a universal journalistic code written down.
An unwritten rule is something like is more akin to a guideline that journalists should follow but don't necessarily have to. Breaking these guidelines, however, has made journalism corruptible and corrupt, and in return a lot of the news that is being reported has been so with a slant, or have been hit pieces targeted by the owners of major papers, rather than real news.
The rules of journalism are "unwritten" although they are taught and spoken. There is not a state journalist's license that I am aware of. You follow the rules to protect your and your colleagues' credibility, not to keep from losing your license. And some journalists keep right on working, despite having been caught plagiarizing or making stories up. Sometimes repeatedly. So, in the sense that the rules are not regulations that you have to follow to keep a license, they are unwritten.
The acceptable practices, policies and rules of journalism are not unwritten. These canons are generally referred to and known as the "ethics of journalism" and they appear in written form as the statements of policy and conduct drafted and ratified by professional journalism associations and as statements of conduct adopted by the various broadcast, print and online news organizations.
It is not good to have unwritten rules serving as the rules of any field or profession. This is especially true for a profession as important or influential as that of journalism. When journalists do not accord themselves professionally and with clear ethics, things can get misreported. Misinformation bought by the public from a supposedly trustworthy source can destroy personal lives, companies and even nations.