When this is non-nil, all functions deleted by the treeshaker are replaced by small stub functions. When a deleted function is called by the application, its stub prints a message telling you that the function has been deleted and how it can be reinstated. These stubs can take up a lot of space if you smash large packages, but are invaluable while refining delivery parameters.
For instance, if your application calls complexp after delivery with :keep-complex-numbers set to nil, a message like the following is printed:
Attempt to invoke function COMPLEXP on arguments (10).
COMPLEXP was removed by Delivery keyword :KEEP-COMPLEX-NUMBERS NIL.
Try :KEEP-COMPLEX-NUMBERS T.
LispWorks Delivery User Guide - 15 Feb 2015