Good point. Now that I think about it, this key as I implemented it so far always gets set when an upstream filter called Modified(), even when INCOMPLETE_TIME_STEPS is not set.
So maybe I should either set it in this context and rename it WIPE_INCOMPLETE_TIME_STEPS_CACHE, or leave the code as is and rename it UPSTREAM_CALLED_MODIFIED so this key is available if someone needs this information for another reason?