Killing the ignition switch will not hurt the alternator unless it drives a relay on the B+ / Alt output wire that opens. These are very, very rare. On many dumb regulated alternators it may not even shut it down because it can remain "excited" even without the ignition which gives it its initial 12V pulse to get going.

Killing the ignition switch on a Balmar regulator shuts down the regulator properly (if it is wire correctly) by removing 12+ from the brown/ignition power wire. By turning off the key the regulator very quickly writes some data to memory, then shuts down the field, than goes to sleep. The ignition switch shuts down the regulator and de-energizes the field wire and the alt can no longer output power once there is no field output to the alt. Cutting the field is the proper way to depower an alternator.

In contrast if you open-circuit the alt B+ while it is loaded, by removing the battery bank from the alternator, from flipping a battery switch through OFF, the voltage spikes instantaneously, to often well above the safe level for the diodes, and poof they go. In theory the regulator would be fast enough to reduce the field output but it is not that fast. They are usually blown in fractions of a second. I do about 6-10 blown diode repairs each year and they are almost always related to battery switch events. Sometimes not all the diodes blow and the alt still marginally works, but at a significantly reduced output.