Here's a more detailed breakdown:
According to it, the total number of crew fatalities in TOS was 59, and 73% of those were redshirts. In the first season, there was a pretty even mix of fatalities in different departments, but in seasons 2 & 3, the fatalities were overwhelmingly security guards. So there is some validity to the "myth" after all.
EDIT: Although comparing it to that Ex Astris Scientia article (we cross-posted), it looks like they counted all the "unknowns" as redshirts, so clearly that skews the results. So I'd say the EAS breakdown is more reliable.