Okay, now it appears that the single data disk “feature” for RAID-Zx is NOT tied to RAID-Zx expansion.
My current desktop does not have the RAID-Zx expansion feature, which is in OpenZFS master, (>2.2.5). I’m using 2.2.3 and I was allowed to create a 2 disk RAID-Z1 pool. Plus, was able to offline one of the drives and yet still have full access to the pool;
root@me:~# zpool status newpool
pool: newpool
state: DEGRADED
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: scrub repaired 0B in 00:00:08 with 0 errors on Fri Aug 23 17:52:44 2024
config:
NAME STATE READ WRITE CKSUM
newpool DEGRADED 0 0 0
raidz1-0 DEGRADED 0 0 0
/disk1.sparse ONLINE 0 0 0
/disk2.sparse OFFLINE 0 0 0
errors: No known data errors
Next, I re-added the sparse file I had OFFLINE and removed earlier. It re-silvered just fine;
root@me:~# zpool status newpool
pool: newpool
state: ONLINE
scan: resilvered 2.33G in 00:00:10 with 0 errors on Fri Aug 23 17:57:02 2024
config:
NAME STATE READ WRITE CKSUM
newpool ONLINE 0 0 0
raidz1-0 ONLINE 0 0 0
/disk1.sparse ONLINE 0 0 0
/disk2.sparse ONLINE 0 0 0
errors: No known data errors
So, I guess single data disk RAID-Zx were always an option.