ZFS can't remove special vdevs from RAIDZ pools

It seems that ZFS cannot remove special vdevs from any kind of RAIDZ pools as of OpenZFS v2.1.4. This behaviour is believed to be a specification being caused by a limitation of top-level vdev removal, I guess.

Suppose the pool which consists of a RAID-Z1 vdev and a mirrored special vdev exists.

# zpool status ztank
  pool: ztank
 state: ONLINE
config:

        NAME        STATE     READ WRITE CKSUM
        zdata       ONLINE       0     0     0
          raidz1-0  ONLINE       0     0     0
            da1     ONLINE       0     0     0
            da2     ONLINE       0     0     0
            da3     ONLINE       0     0     0
        special
          mirror-1  ONLINE       0     0     0
            da4     ONLINE       0     0     0
            da5     ONLINE       0     0     0

errors: No known data errors

Now, run zpool remove command to remove the special vdev, and an error occurred like below.

# zpool remove ztank mirror-1
cannot remove mirror-1: invalid config; all top-level vdevs must have the same sector size and not be raidz.

I think that removing top-level vdev “mirror-1” all at once is not good, then try to remove each member of the vdev, but no sense.

# zpool detach ztank da5
# zpool remove ztank da4
cannot remove .....

By the way, it is possible to remove slog or L2ARC which is one of top-level vdev kinds from any RAIDZ pools for some time, and besides, non-RAIDZ pools have no limitation to remove top-level vdevs.

I have been preparing to apply a special vdev to my RAIDZ2 pool on real part, but it is a little hard to do it. It was good I tried these beforehand on a VM… L2ARCs and slogs are removal from any RAIDZ pools so special vdevs will become possible to do in a future or else it won't because the special vdev removal have to be strictly evacuated data in the vdev to main vdev?

I will have no choice but to alternatively use p2L2ARC for the time being.