I really dislike the current use of red flag. Seems it's used for any personality trait one doesn't like instead of being an actual red flag. I think people who freely use the term red flag are judgmental and thats a big...red flag...
What do you think red flag means other than incompatability? It's not like the usage has changed, it's always been meant to signify "I don't like this about you".
I don't disagree with you about judgemental people overusing it, though. But that's just judgemental behavior in general, I think you might just not like judgy people - which is fair.
I use to think a red flag was a telling sign that someone isn't who they say they are, borderline psycho or have other underlying issues such as being abusive or manipulative.
I worded that poorly but I said it was a telling sign, like a warning. Maybe the person spreads rumors or compulsively lies. Granted, it's always difficult to spot signs of true manipulators when you are the victim. But you're probably right though, it might just be my interpretation.