The reference should be used in conjunction with the Qwt manual on your hard disk or Qwt on the web. Only the differences specific to the Python bindings are documented here.
In this chapter, is not yet implemented implies that the feature can be easily implemented if needed, is not implemented implies that the feature is not easily implemented, and is not Pythonic implies that the feature will not be implemented because it violates the Python philosophy (e.g. may use dangling pointers).
If a class is described as being fully implemented then all non-private member functions and all public class variables have been implemented.
Undocumented classes have not yet been implemented or are still experimental.