The API contract laid out in their documentation. There’s a screenshot in the post right around there.
The API contract is:
> What happens when an author deletes their Tweet?
The Twitter widgets JavaScript will not display a fully-rendered Tweet if the Tweet no longer exists on Twitter. The fallback <blockquote> containing Tweet information will be visible on the page.
The API contract is:
> What happens when an author deletes their Tweet? The Twitter widgets JavaScript will not display a fully-rendered Tweet if the Tweet no longer exists on Twitter. The fallback <blockquote> containing Tweet information will be visible on the page.