It is suspected that Reddit has been served with an NSL. National Security Letters (NSLs) are subpoena served by the United States federal government and often come with a gag order that prevents the recipient from even stating that they received the letter.
Warrant canaries are used to circumvent gag orders by stating that requests have not been received, under the legal theory that, while courts can compel persons not to speak, they can’t compel them to lie. Reddit’s canary has died—the canary is absent from their most recent 2015 transparency report, where it was present in the 2014 report.
Does this mean that you should stop using Reddit? No; canaries are an important transparency method. If you are worried about your privacy, you shouldn’t disclose the information to a third party to begin with. Note that this includes metadata that are gathered about you when you, for example, browse subreddits while logged in. You can help mitigate that by browsing anonymously using Tor, being sure never to log in during the same session.
The website Canary Watch is a website that tracks warrant canaries.
I’m awaiting further analysis after the weekend.