Is there a proper convention for applying and removing phantom power to a microphone, so as not to damage the mic? Can the phantom supply be on when plugging or unplugging a mic? Should the phantom power be turned on after the mic is plugged in? Does it matter?
First, we assume the microphone is low impedance and balanced like all professional mics. That being the case, you will not damage a microphone regardless of how you do it. You do not need to worry about turning off the phantom power before plugging in a mic.
But, you could damage your loudspeakers if your mixer is turned up when you plug in a microphone. This often results in a loud "pop". Turn the mixer's Master control down before connecting or disconnecting any microphone. This will prevent damage to your loudspeakers.