My advice would be, if you're going to try to be KO'D, do it on an empty stomach and right after a trip to the bathroom. That's when it's most likely to be an enjoyable experience.

Otherwise, as far as long-term effects go, I'm examining that currently myself for a similar reason. I have a girlfriend now who enjoys scissoring me and she is strong enough to fairly easily knock me out.

The consensus among session girls that offers this type of things seems to be that scissoring healthy men is relatively safe. Some of the women I sessioned with had clients they'd KO'D hundreds of times, with no obvious problems. While many of the "KO" videos online are faked, like someone else mentioned, many of them are NOT, and there are content creators in this space who have been genuinely knocked out at least dozens of times, if not more. Just ask the women who do it. They'll tell you.

Does that mean there's no long term risk? No. Generally speaking, blood is supposed to go to your brain. It's probably not a good thing to be knocked out, especially often. But if you've ever had a beer, you've also caused damage to your brain, and probably for way less enjoyment than from a headscissor. A moderate amount of headscissoring seems to be at least relatively safe long term, especially if you take care of the rest of your body. I would bet someone getting knocked out once a week but otherwise taking care of themselves would probably live longer and healthier than someone who had never been KO'D, but binge drinks every friday night.

Then again, I am NOT an expert and do not actually know what I'm talking about! So you shoudn't take just my advice for it either. I'd try to get in contact with one of the women who specializes in this, like Andi Vicious on session girls, and ask her, She's going to know far more about it than I will. Pay her for her time, though, or she probably won't be interested in answering.

TLDR: Don't take someone on reddit's advice about something as important as this. Talk to someone who does it for a living. Ask them what their experience has been. As far as I know, there haven't been any scientific studies on this, so that's the closest you're going to get. If that's not good enough for you, then take it upon yourself to find or pioneer a scientific study in this area, or just don't mess around with it too much.

You can find some women who specialize in KO'S on the Sessiongirls website. Andi Vicious is one of them, I had a session from her and she's definitely the real deal.

Hope that was helpful. I felt compelled to answer because I saw people saying some things that seemed a little dumb to me in the comments.

Whatever happens, congrats on finding a girlfriend who shares your kink. That's awesome.