A lot of Chromebook and Chromebox users don't realize this, but all ChromeOS devices have an expiration date.
Google's original policy was for devices to be supported for five years, but the company has recently extended that time to 6.5 years. When your Chromebook or Chromebox approaches its built-in expiration date, it will warn you that it's time to go buy a new device entirely. Not long after that, it will refuse to apply any further security or feature updates. In addition to leaving users vulnerable to unpatched security exploits, this means that constantly evolving services such as Gmail will eventually stop working entirely.
Google has been working on a way to warn users six months ahead of time that their device's EOL date is approaching to allow them to plan with a little less time-sensitive desperation. But users running the Canary and Dev early-preview ChromeOS builds discovered a bug in the new code the hard way. After any reboot, brand-new devices started warning "this is the last automatic software and security update for this Chromebook. To get future updates, upgrade to a newer model." Luckily, the code had not yet made it to stable—or even beta—builds, so few users were exposed to it. The bug has already been patched, and users on Canary should already have seen the bogus message disappear after a daily upgrade push this past weekend. Users on the Dev build only get updates weekly, so they might not have the warning fixed until sometime this week. On the other hand, if you're not running a Canary or Dev build and you've gotten a message about your device being beyond its support date and getting no further upgrades... it's legit, and you really won't be getting any more upgrades.