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1. A 90% failure rate (and yes, I looked into it in the past and that's the number quoted by multiple professionals who can cite research on the topic) isn't a good enough chance to be worth getting a piercing I don't want in the hope that it might help.
2. Partly untrue, and partly irrelevant to the reasons why it wouldn't help most people with migraines, and yes I've tried it and it doesn't help me.
3. That's nice. When I was still testing things, I found less than 2% of my library was safe to launch without the client, and it's a demonstrable fact that there are more recent games which can't be run without the Steam client than ones which can. It's entirely possible for someone not affected by the issues I've had to deal with to not notice when it opens as a background app, while someone whose condition(s) get(s) triggered by the client will noticed because they end up in pain/needing medication/getting an ambulance called to take them to hospital shortly after opening the game.
I've done plenty of testing of lots of possibilities which haven't stopped Steam from being harmful, and plenty of investigating of many more which I didn't feel were reasonably-priced or unintrusive enough to be worth testing personally. But, again, I do appreciate that you're approaching this with the intent to be helpful. It's not helpful, unfortunately, but I can tell that you want to help, so thank you for that much at least.
Just for full disclosure: Haven't heard any claims of daith piercings coming with any risks, other than the high probability of not having any effect. I have preventative medication with very minor side effects (totally ignorable if I have them in the morning and drink lots of water) as well as painkillers which the preventatives mean I only need maybe once a month instead of my previous frequency being comparable to your pre-piercing results.
I've heard of claims that it's a placebo, but been told it probably isn't, and that in the cases where it works, there's good reason to believe it's making a real difference, not just a psychological effect. Glad you've found something that works for you, and I do think it's worth recommending/considering for others with migraines, even if I don't think it's a good choice for me personally :) so, again, thank you!