Raffaele T., TuxPhones:
In spite of being, technically, based on the Linux kernel, Chromebooks were traditionally seen as very difficult devices for the free operating system, since proprietary drivers for even basic components like keyboards were often designed to discourage any kind of modification of the OS.
This, however, is mostly a story of early Chromebooks. In later years, projects like GalliumOS focused on these increasingly popular devices, and while still difficult, installing or dual-booting Linux became a choice for these Google machines.
In fact, since some years Chrome OS's own crostini ("bread crusts" in Italian?) feature provides a "Linux subsystem for Chrome OS", which allowed installing most Gtk and Qt apps alongside Android ones. This is, however, internal to Chrome OS, and similarly to Microsoft's WSL, it did not really open the OS in any way.
Now a new project, Cadmium, aims at providing a stable (and mainline) Linux experience for selected Google Chromebook devices.