This is consistent with removing whole-disk vdev support from libsa/zfs/zfs.c in r342151.
What I'm really after is to not probe for a whole disk vdev if a partition table exists, so if for some reason whole disk vdev support needs to remain, I'll chase this patch with one to check for a partition table first and only do a whole-disk vdev probe if there isn't one.
The reason I want the partition table check to take priority is that I had an experience recently where I moved a machine with a gpt/zfs config to 11.2, and it 'started to hang' in zfsloader. After then upgrading the boot code to the 11.2 gptzfsboot, gptzfsboot 'started to hang'.
After a bit of digging, it turns out what was really going on was that the RAID controller the disks were attached to (and which was set to logical pass-through mode) had a firmware bug that was being tickled by the new zfsboot/loader code in 11.2 that newly started looking at the labels at the end of the disk (or partition), and once that happened, each I/O was causing a 60 second timeout (disk size was accurate - no past-the-real-end issue, and it turns out I could get things to work by breaking up multi-sector I/Os near the end of the disk into single-sector I/Os in descending order). This particular problem was worked around in the end by updating the controller firmware to a version that fixed this bug, but if no such firmware existed, I sure would have liked to have been able to work around the issue by adjusting my partition tables to avoid a suitable chunk of the end of the disk. Such a workaround is not possible unless partition tables take priority over whole-disk probes (or whole disk probes are done away with entirely).