When editing a widget, if a widget has been retired you will see a pop-up with the following message:
"This widget has been retired.
You can keep using it. However once removed, it cannot be added again. Please use the latest version or contact support to find out more."
Widgets are retired when there are newer ones in that category, with updated functionality.
A new widget can be added to the page, and the retired widget can then be deleted.
See the article Adding a widget in Web Builder for instructions on adding a widget.