In the classic Basecamp API, there is a "deleted" attribute or field in the Person model; see reference page here
Is there a corresponding attribute in the "new" Basecamp API? If not, how can I can a list of users who are no longer in the account or project?
There isn't an equivalent in the new Basecamp API - making a request to /people.json will return all current users that the authenticated user can see. We don't currently include deleted people - I'll pass on the suggestion :)