I'm guessing that it's because of the clip of that old man getting pushed over. I can actually see why that would warrant a content warning for people who are sensitive to seeing real life people bleed. I think that the dude died too so a warning for that is probably good for people who are uncomfortable watching a man literally die.
Granted, I think that people NEED to be uncomfortable and outraged, because the treatment of that man was inhuman and maybe some people need to realize how bad it is...
If you're interested to get back into it then I would suggest 당신 to also watch the two seasons that were released before the Netflix version since they show a lot of stuff that is important in order to understand the series better.
