Google's Chrome OS version 63 is beginning to roll out to some Chromebooks, according to users who started reporting the update as early as December 14. Via the source, which follows a thread posted to Google Plus, some of the devices to have already seen the update include the HP 14 Chromebook, Toshiba Chromebook 2, and the 2013 model of Google's own Chromebook Pixel. As with other rollouts, this update will likely be coming to other Chromebooks as well over the coming days and weeks, with some taking longer depending on the device's manufacturer.
Breaking from what normally happens with an update though, this update is not said to have included a changelog for Chrome OS specifically and the official Chrome Release Blog only highlights changes brought to the Chrome browser itself. The changes to Chrome OS should follow those implementations, which are also listed through the official Google Git for Chrome, fairly closely, though they don't provide any insights into what new features might come directly for Chromebooks. Unfortunately, that means that as of this writing, all that can be verified with the update is that it likely incorporates several bug fixes and optimizations - in addition to the usual security updates. Specifically, the stable channel update for desktops on Google's Release Blog says that Chrome version 63 brings 2 security fixes, including a high-severity bug listed as "High CVE-2017-15429: UXSS in V8," though no further description is provided and the company lists a reward to the vulnerability's finder of $7500. Aside from that inclusion, the update to Chrome is said to include fixes to problems found from "internal audits, fuzzing and other initiatives." It bears repeating though, that those are not marked as being specific to Chrome OS, which has a degree of separation from the Chrome browser. So, those fixes are not necessarily a part of the as-yet-unclarified Chrome OS update.
It is, admittedly, strange that Google would deliver an update without a changelog and that it would subsequently take down the associated release blog, there is a good chance that will be reposted to that site in the future. In fact, in all likelihood, the oddities are the result of an error that was somehow made somewhere in the update's release process. So there's a reasonable chance that will be rectified in the near future. In the meantime, Google has also recently been reported to be working on the next version of the operating system, namely Chrome OS 64. That iteration has already begun rolling out to some users on the beta channel of the OS and is reportedly delivering better security measures with regard to redirects, among other things.