As a default, the public archive is turned on for all TinyLetter accounts. To change this setting, go to Username > Your Profile, scroll to the bottom of the page, and check or uncheck Show sent messages on your subscribe page.
You can choose to have your newsletter show up in your archive by selecting Make letter public when you compose a message. As well as disable the archive for newsletters after they’re sent. Do this by going to Sent, selecting your letter, and choosing your preference for Show in Letter Archive.