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There is a newer version: 4.23.0.17664

This rule raises an issue when a private nested class is never used.
Why is this an issue?
"Private" nested classes that are never used inside the enclosing class are usually dead code: unnecessary, inoperative code that should be
removed. Cleaning out dead code decreases the size of the maintained codebase, making it easier to understand the program and preventing bugs from
being introduced.
Python has no real private classes. Every class is accessible. There are however two conventions indicating that a class is not meant to be
"public":

   classes with a name starting with a single underscore (ex: _MyClass) should be seen as non-public and might change without prior
  notice. They should not be used by third-party libraries or software. It is ok to use those classes inside the library defining them but it should
  be done with caution. 
   "class-private" classes are defined inside another class, and have a name starting with at least two underscores and ending with at most one
  underscore. These classes' names will be automatically mangled to avoid collision with subclasses' nested classes. For example
  __MyClass will be renamed as _classname__MyClass, where classname is the enclosing class’s name without its
  leading underscore(s). Class-Private classes shouldn’t be used outside of their enclosing class. 

This rule raises an issue when a private nested class (either with one or two leading underscores) is never used inside its parent class.
Code examples
Noncompliant code example
class TopLevel:
    class __Nested():  # Noncompliant: __Nested is never used
        pass

Compliant solution
class TopLevel:
    class __Nested():
        pass

    def process(self):
        return TopLevel.__Nested()

Resources

   Python documentation – Private Variables 
   PEP 8 – Style Guide for Python Code