Github Issue #885 - qubes-dom0-update could check whether /boot is mounted

It is probable but I believe this scenario to be highly unlikely. A user who is using split boot with detached LUKS header is an advanced user. They are probably following a proper guide such as your community guide or know what they are doing.

The proposed patch might not help the unlikely scenario, but will neither create further issues. I guess the patch could be tested for one or two weeks and then submitted for review.