It should be said though that only Pixel, Fairphone, and maybe some Motorolas support relocking the bootloader with a custom OS.
Without that ability, anyone can plug in to your phone and write whatever they want to the internal flash and your phone will be none the wiser.
Happens to me all the time. Also on laptops, secureboot is such a life saver any time you're out and about
...it's sure nice this exists and is available to anyone but it's not seriously a risk if you're not of interest to people who are willing to physically show up and bug your hardware in a way that requires quite a bit of preparation
The OnePlus6T supports it, it will still display a warning but it will be locked.