In Product settings → Attributes, at the very bottom of your list of attributes is an additional dropdown named "Archived attributes" that lists every attribute that was previously archived.
From the archived list, choose the attribute you want to restore, select "Restore attribute," and save the changes. Once the attribute has been restored, it will be moved back into your attributes lists, and you will be able to use it in your workspace.
Restoring an attribute option
Similar to restoring an archived attribute, attribute options that have been removed can be restored as well. From the "Removed options" dropdown, select the option you want to restore, save the changes, and voila!
Happy tasking 😃