Most disk shelf components are hot-swappable; you do not need to remove power when replacing them. Do not remove a component if you do not have an immediate replacement. The disk shelf must not be operated without all components in place.
If a disk shelf is a part of a storage pool, powering off or removing all SAS chains from a disk shelf will trigger a kernel panic on the controller(s) to prevent data loss, unless the shelf is part of an NSPF (no single point of failure) data pool. To avoid this, shut down the controller(s) before decommissioning the shelf, as described in Powering Off the Controller.
For details on NSPF profiles, see Storage Pool Concepts in Oracle ZFS Storage Appliance Administration Guide, Release OS8.8.0