Deactivating your account permanently removes your profile and information from Twitter. Deactivated profiles will take 30 days to clear from our internal systems. While the deactivation is being completed, usernames or email addresses cannot be used in the creation of a new account. Before you do it, you should know...
Note: we change your settings immediately, and any/all old links appearing in a Google or other search index thus lead to error pages. However, these old links appear because Google and other search engines have cached old content in their search index. Twitter has no control over this, but you can send them a request to take it down.