This is a variation of Technology Disclosure. The technology exposed in this finding is the operating system running on the web server.
If the version of the operating system is disclosed as well it will be indexed in specific search engines. It is therefore a safe bet to say someone will launch an automated attack against the server soon after a vulnerability in the version has became publicly known.
If the version is not disclosed, but just the name of the operating system, this is not as dangerous but still something that can aid an attacker in further attacks.
What can happen, example and remediation?
Please see the article about Technology Disclosure. As this finding is just a variation of that, all the details are the same.