Yes it is standard for extended Jenkins items. But generally, listener does not have actual implementation.
This listener is used to add listen feature to Jenkins items. If listener is abstract class, extended Jenkins items cannot add listener to itself. So this should be interface.

Don't worry, Jenkins.getExtensionList(class<T>) can be called with interface class.
In addition, I have seen this solution in other plugin. (Sorry, I forgot what plugin..)