I've been watching star trek (TNG, DS9, Voyager) and still don't get the red shirt reference. I know it's about them getting killed all the time but I just don't see it.
Obviously the one who is not one of the lead actors, whose character usually doesn’t have a name, no lines, never appeared, and is from security and tasked to protect the captain and jump first in combat is going to survive.
The meme is from TOS. Not as many people died in the newer ones.
A simple pie chart reveals 55 total deaths and, yes, red shirts perished in frightening numbers. A whopping 24 died, compared to 9 in yellow/gold command and 7 in blue, with 15 crossing into the final frontier in unidentified colors
In TOS, there were more characters in red shirts than any other color. So of course most deaths were characters wearing a red shirt. Proportionally, I believe it was actually yellow with the highest mortality rate.