I think the issue with head kicks today is that the standing leg can't be targeted in a lot of martial arts sports as defined by the rules
1. You can't target the groin
2. You can't do a back sweep on the standing leg
3. You can't kick into the knee of a standing leg
The rules in martial arts sports pretty much protect the front leg from sweeps, with the exception of the front sweep. Because of the way the rules are set up, people have an inaccurate perception of the high kick.
Accidental kicks to the groin when doing a side kick is considered to be "an accident" and against the rule in martial arts sports,
But in real life application. That's exactly what you want to do and those kicks to the groin are actually good martial arts; the way it should be without the rule set.