Earlier quoted context omitted.
Definitely an urban legend at this point. A reddit thread[0] mentions it being collected in 1990 from computer stories going around in the 80s[1] [0] https://old.reddit.com/r/talesfromtechsupport/comments/5yrs1... [1] https://www.goodreads.com/book/show/3227607-the-devouring-fu...
The version that I remember hearing was https://www.snopes.com/fact-check/polished-off/ . Also false.
But, the key thing is that urban legends always reflect some sort of underlying societal belief or worry. "Janitor polishes off patients" is about the fear of death, and how it can come for anyone at any time, and there's nothing they can do about it. "Janitor unplugs server every night at 5" is probably more about the idea that strange things happen sometimes, but it usually turns out that there's a good explanation.