Is it not yet completely insane for someone who engages in combat a lot to have long, say shoulder-length, hair? I saw your posts about armour and put two and two together when you said anything that can be grabbed isn’t that good of an idea, so hair…?
This one’s pretty simple; you’ve identified one of the two main issues.
So, long hair is a detriment to combat effectiveness, but it’s not terminal. You don’t want hair that will get in the way, and you don’t want hair that can be grabbed. After that, do what you want.
So, long hair will, usually, get in the way. No matter your best intentions, it’ll fall in front of your face, get in your eyes, and, generally, make a nuisance out of itself at every opportunity. Before someone tries to come down on me for this: I’ve had long hair for most of my life. I know.
The simplest answer is: You bind it back in a tight ponytail; that will keep it out of the way. Historically, this has been men’s fashion in a number of cultures. Ideally a shorter ponytail is better than a longer one, but to some extent, it doesn’t matter. Buns are also an option, as are dreadlocks and tight braids. Anything that keeps the hair out of the way will do the job.
So, why bother? Because in colder climates, long hair will keep your head warm. Hair is a fantastic insulator, and if you’re living on a tundra, that will help keep your head warm when you’re out working. When you’re fighting, you’ll probably want to tie it back.
The other reason is, yes, someone can grab you by your hair. There’s a couple caveats here, and they depend on the kind of combat you’re in. In a fight with bladed weapons, going for someone’s hair is a good way to get carved up. You’re giving up one hand, and limiting your mobility, and hoping your opponent won’t use one hand to grab your weapon arm, and the other one to run you through repeatedly. This is less of a consideration if your foe is wearing armor, though, at that point, you probably are as well, and getting into the situation in the first place is going to be harder.
Also, ponytails blunt the effect of having your hair pulled. I suspect this has something to do with how it distributes the force across the scalp, but a ponytail will not offer the same control grabbing a fist full of loose hair will.
In ranged combat, somewhat obviously, you’re not going to be getting a grip on someone’s hair, and some the same considerations apply as the previous example. If you do get in close enough to grab their hair, they’ll probably pump a couple rounds into you.
Not, strictly related, but long hair can be a detriment when working around heavy machinery. If it gets caught, it can get pulled in. So, that’s something else to avoid. At that point, even the ponytail can be serious safety issue. This is more of a general, “yeah, long hair can be a detriment, than a specific issue, and shoulder length shouldn’t be a problem.
So, no, nothing insane about having hair, that’s normal, and long hair is common enough, but it does come with special considerations.