The lunate sigma was the standard form of the sigma in old uncial styles of writing, and in early Byzantine hands. By the time the lower case letters reached the form we still use today the medial/final distinction was basically in place. Some editors (and at least one Textkit regular) seem to feel that the sigma, out of all the letters of the Greek alphabet, needs to be reformed.
You’ll only see it in older manuscripts and in some texts printed in the last 40 years or so. We have debated the subject on Textkit several times before, 5009, 3573.