As the AO3 software has been stable for a long time, the change is mostly cosmetic and does not indicate that everything is finalized or perfectly working. Exiting beta doesn't mean we'll stop continuing to improve AO3—our volunteer coders and community contributors will still be working to add to and improve AO3 every day. For one, it’s likely you’ll continue to see references to us being in beta for a while as we update our documentation.