Comment by charcircuit

>and you may damage your device permanently

https://service.oneplus.com/us/search/search-detail?id=op588

They make it clear that this feature is unsupported and it's possible to mess things up. The reason why it's an ideal and not an expectation is that flashing alternate operating systems is done at one's own risk and is unsupported. They have already told the users that they bear no responsibility for what may go wrong if they flash the wrong thing on that device. Flashing incompatible operating systems to the device requires people to be careful and proper care to ensure compatibility before going through with flashing was not done.