I agree we could do a better job at publicizing the logic behind these changes, and we will in the future.
Note that this change isn't intended to remove any functionality, it just removes the aliases that we put on the Deferred object returned by $.ajax so that it uses the standard Deferred names. It should be possible to move to the new names via a simple text replace in most cases.
@eli, no there is not a roadmap for removal in general. We are deprecating things to indicate that they are not the best-practice way to do something in jQuery. It's possible that things that are deprecated are *never* removed, but then again we are unlikely to add features to it or fix bugs in a feature like that.
success, error,and complete handlers should be aliased for backward compatibility for an extended period, if not indefinitely
Agreed about the extended period. Note, however, that even if we wait five years there will be people who have not changed their code. Deprecation is the start of the notice we need to give if we are ever to have a hope of removing something.
In the meantime, nothing prevents anyone from using the features marked deprecated in 1.7, including this one. We will start a separate (and more visible) thread about the direction of jQuery once 1.7 has shipped.
could we release a compatibility plugin
That has been suggested and would most likely be our approach for many of these things in the 2.0 time frame.