In Taxi, you can see what Projects, Mailings and Versions are using an Email Design System through the Usage page in the Email Design System settings.
To access the Usage page:
Select Email Design Systems from the menu on the left of your Taxi screen
Select the Email Design System you want to check the usage of
Select the Usage tab
You will see all the Projects, Mailings and Versions listed that are using this Email Design System and you can open and edit from this page too.