I agree. SLF4J ( with logback ) does not require any code dependencies to the
underlying logging framework. The factory provides logger instances that
implement SLF4J interfaces.

Why be belligerent about the request to put a shutdown interface into the API
(Even suggesting its a feature)? If you would add the method/interface, the
implementation of the logger can do what they want with the interface, and the
user of SLF4J (me) can invoke it at the proper time in the application
lifecycle.

What you are suggesting eliminates the value of having logger agnostic API to
code to.  I mean really? get the underlying logger, cast it to an
implementation, and then use it? If I am building a code dependency on the
underlying logger, why even use SLF4J? Come on guys.