A list of packages whose conditions are removed (that is where the symbol-package of the name of the condition is one of the packages). The system automatically adds the internal packages to this list. Conditions that are in these packages but are also in the :keep-conditions list or its precedence list are kept. The defaults cause all the conditions that are defined by the system and are not standard to be deleted. To keep all the conditions, you should pass :keep-conditions :all (or :condition-deletion-action nil). To eliminate all conditions, you should pass :keep-conditions :none.
See Deleting of condition classes.
LispWorks Delivery User Guide - 10 Aug 2017