I thought this no longer works with the T2 chip? Don't you run into issues with e.g. upgrading the firmware because it will only accept firmware updates from the internal drive?
I got a brand new mac with a T2 chip this week from my work. The first thing I did was do all the updates, including a security update for the OS, which required a restart. During the restart it was giving errors, so it was not able to boot properly. It would just give the error message and you could only restart again, just to have the same thing happen. A colleague more familiar with macs, brought up a menu, where you could choose which drive to boot from, which eventually fixed the problem, since it booted from the hard disk. In my case this menu had only the internal ssd, since I did not have anything else connected. I would guess that if I had an external hard drive it would show up as well.
Also on the documentation of the T2 chip, Apple strongly recommends to use file vault, because if you do not, the ssd even though it is encrypted by default, gets decrypted [1] upon being mounted.
To sum up, I guess it is possible to boot from whatever device you want.
Also on the documentation of the T2 chip, Apple strongly recommends to use file vault, because if you do not, the ssd even though it is encrypted by default, gets decrypted [1] upon being mounted.
To sum up, I guess it is possible to boot from whatever device you want.
[1] https://support.apple.com/en-us/HT208344