Decommissioning a host from a vSAN cluster satisfies all the criteria described in article Permanently Decommissioning a node from vSAN Cluster.
SDDC is unable to remove the host from cluster followed by this error:
The task from SDDC is designed for a host that is powered down.
This is an expected behavior if the host being removed from SDDC is still powered ON.
If the host is in powered ON state, place it in maintenance mode with full data migration and shut it down. Re-try the task 'Remove host from cluster' from SDDC again.