Chromebooks built on Linux 3.14 kernel or older will most likely not be receiving support for Linux applications, based on a new commit spotted in the Chromium Gerrit. That's because Linux containers and virtual machines, in addition to associated VSOCK and security patches, won't be backported to those devices. In short, Linux apps exist in what are essentially packets of runtime elements, app services, and supporting files, referred to as containers. They can also be run in virtual machines. For Chromebooks and other devices in the Chrome family, VSOCK facilitates communication between those and Chrome OS. Without those in place, Linux apps aren't going to be possible. So, although not confirmed by Google at this point, the implication seems to be that the Linux kernel is the dividing line due to difficulties associated with backporting everything required to run the apps.
Thanks to a list of Chrome OS devices maintained at the official 'The Chromium Projects' website, it is possible to get some idea of which devices won't be supporting Linux applications. The site currently includes 98 Chrome OS devices and various information, including Linux kernel versions. The list includes the Acer Chromebook 13 (CB5-311), Acer C670 Chromebook 11, Acer Chromebook 15, ASUS Chromebook Flip C100PA, ASUS Chromebook C201, Dell Chromebook 13 (7310), Google Chromebook Pixel (2015), HP Chromebook 14 G3, and Toshiba Chromebook 2 (2015). It isn't just Chromebooks that won't be receiving support for Linux apps either. Acer's Chromebox CX12, Chromebase, and Chromebase 24 won't be able to run the apps either, in addition to AOpen's Chromebox Mini and Chromebase Mini. Meanwhile, both ASUS's Chromebit CS10 dongle and Chromebox CN62 and Lenovo's ThinkCentre Chromebox will be excluded as well.
In the meantime, Linux apps are expected to arrive alongside the incoming Crostini update in Chrome OS version 69. That has also been marked as "(Beta)" in the associated update plans, so there may be at least a few unforeseen bugs arriving alongside the feature for users that choose to download and run Linux applications. Bearing that in mind, it will most likely be disabled by default within a Chromebook's settings in order to prevent those issues from impacting users who aren't going to use those apps. Once turned on, users will be able to easily download full desktop applications such as Steam or Photoshop, bringing their Chrome OS experience a bit closer in productivity to other operating systems.