Why is the requirement in 110-27, that live parts greater than 50 volts be guarded? There have been fatalities, although small in number at less than 50 volts. So where did 50 volts come from?

See ARTICLE 720 Circuits and Equipment Operating at Less Than 50 Volts

Been in the code for many years, and probably was the basis for the voltage indicated.

50V has also been a threshold in some parts of the British "code."

I was told that 50 volts came into the code because 48 volts is what is used for telephone/communication circuits.

I think it's more to do with the calculation of the likely maximum current available with 50V based on "average" body resistance (if there is such a thing, based on another thread).

The DC supply for telephones is generally 48 or 50V, but the AC ringing supply can be 90V or more.