content
is a namespace in your example. Your element name is actually encoded
so you will need to use the attribute marking your property as such:
[XmlElement("encoded", Namespace => "custom-content-namespace")]
public string Content { get; set; }
Note that you will need to declare the namespace in your containing XML:
<content:encoded xmlns:content="custom-content-namespace">![CDATA[...
This also means any child nodes would be prefixed with the same namespace. Not so much an issue for CDATA
content, but just in case you have other elements you are trying to deserialize.
For a related questions to this, see Deserializing child nodes outside of parent's namespace using XmlSerializer.Deserialize() in C#