OnePlus has no such restrictions that I've seen, from the One to the 8T. I haven't heard of them locking down newer ones either. You just run the usual command to unlock the bootloader, no funny business. I've run a custom ROM on all of them that I've owned.
OnePlus also doesn't allow enrollment of custom signing keys like Pixels do so they're objectively worse for your ownership even if they don't phone home.
Their security patch and update support for hardware is also abysmal.
