There are a lot of other devices (that aren't x86 PCs) which let you boot OSs wholly from third parties. A couple that come to mind are Canon Cameras[1], and the Remarkable Tablet[2]. A lot of routers, too, don't put up any barriers the installation of alternate firmware (some do), and even Apple Silicon Macs can be made to boot unsigned OSs (and are getting a Linux port). Not to mention the Raspberry Pi and its ilk.

I say this almost preemptively, because I really don't think it should obscure your core point: open devices are increasingly the exception, not the norm.

1: https://chdk.fandom.com/wiki/CHDK 2: https://hackaday.com/2020/09/06/a-free-software-os-for-the-r...