zpool(1M) is able to offline cache vdevs despite man page saying that it isn't:
       zpool offline [-t] pool device ...
           Takes the specified physical device offline. While the device is
           offline, no attempt is made to read or write to the device.

           This command is not applicable to spares or cache devices.
altair:root:~# zpool create testoff c9t67d0 cache c9t71d0
altair:root:~# zpool status testoff
  pool: testoff
 state: ONLINE
  scan: none requested
config:

        NAME        STATE     READ WRITE CKSUM
        testoff     ONLINE       0     0     0
          c9t67d0   ONLINE       0     0     0
        cache
          c9t71d0   ONLINE       0     0     0

errors: No known data errors
altair:root:~# zpool offline testoff c9t71d0
altair:root:~# zpool status testoff
  pool: testoff
 state: ONLINE
status: One or more devices has been taken offline by the administrator.
        Sufficient replicas exist for the pool to continue functioning in a
        degraded state.
action: Online the device using 'zpool online' or replace the device with
        'zpool replace'.
  scan: none requested

At https://github.com/illumos/illumos-gate/blob/master/usr/src/man/man1m/
zpool.1m#L931
     Do not add a disk that is currently configured as a quorum device to
     a zpool.
– should be:
     Do not add a disk that is currently configured as a quorum device to
     a pool.