I am having this same issue. I had to replace a drive in one of my zpools and realized all the drives I have setup for ZFS and passed through the button is now missing. Interestingly enough the new replacement drive has the button option for "passed through" but it is missing from all the others. I only realized this when I tried to take the failing drive out of "passed through" mode.
Also, I have another Unraid server running multiple ZFS pools I just checked but the "passed through" button seems to be there on all the drives of that server.