At least partially - the technical introduction for the Z17 says that several items can be concurrently maintained (IBM-speak for hot-swapped). So far as major items like the processing units - maybe (still reading).
> Four Power Supply Units (PSUs) that provide power to the CPC drawer and are accessible from the rear. Loss of one PSU leaves enough power to satisfy the power requirements of the entire drawer. The PSUs can be concurrently maintained
I would think their customers would demand zero downtime. And hey - if BeOS could power down CPUs almost 30 years ago I would expect a modern mainframe to be able to do it.
(3.4mb PDF) https://www.redbooks.ibm.com/redbooks/pdfs/sg248580.pdf
> Four Power Supply Units (PSUs) that provide power to the CPC drawer and are accessible from the rear. Loss of one PSU leaves enough power to satisfy the power requirements of the entire drawer. The PSUs can be concurrently maintained
I would think their customers would demand zero downtime. And hey - if BeOS could power down CPUs almost 30 years ago I would expect a modern mainframe to be able to do it.