Smart List Used By Custom Column in Report

I created a smart list that is now used as a custom column in a Lead Source report, however when I go to the list, it doesn't show that it is "used by" anything. This has caused a few lists to be inadvertently deleted which then leads to inaccuracies in the reports I'm pulling.

It would be great if "custom columns" were added to the "used by" tab of smart lists.