Each entry contains explanations for why they are in a given category.
Fairphone due to having some online code activation thing for OEM unlocking. Bootloader of my FP4 has been unlocked for quite some time now, but if I recall correctly, this activation step was required for me to load custom ROM on it.
https://support.fairphone.com/hc/en-us/articles/10492476238865-How-to-unlock-and-re-lock-the-bootloader
https://shop.fairphone.com/bootloader-unlocking-code-for-fairphone
This website disappears, you can't unlock your bootloader.
That is why Fairphone is in Caution section.

Microsoft being in "safe" is a bit funny, and the explanation acknowledges that as well.
But it is there due to it containing only two Surface devices, which have easy and standalone bootloader unlock process.

Ah yes, you are correct.
Apparently the url to signup form is https://volunteers.fosdem.org/volunteers/signup

In https://volunteers.fosdem.org/?

It will have unlocked bootloader and quite standard SoC, so you will be able to install PostmarketOS or any other mobile Linux on it.
But in that case you might as well buy FairPhone 5 right now and do the same to it.

Yes, and so is a virtual machine. I'm thinking install Linux to disk so that it can then run directly on hardware.

Yes, this is something that should be taken into account when designing this software.
Set dual-boot as a default / design UI in a way that offers dual-boot as a preferred option.
And many other technical issues will probably appear that will have to be figured out.
But I think that at least even thinking about this is a good start.
Also, this reminds me of 2013, when people accidentally nuked their Windows installs with Linux because they wanted to get the Tux in Team Fortress 2 (Valve gave it to people who played Linux version of TF2).