This is only true of Marketplace plans (which are, increasingly, very bad deals is you're not actively using your insurance - even back in 2015 when I was looking for Marketplace plans for myself, I could easily spend close to that amount on the premiums of plans that had high deductibles/coinsurance/etc. and it's not like the Marketplace has gotten more competitive since then), and I think it's only true of expenses that the insurance company is willing to cover - if you get treatment out of network, or if you get treatment beyond what's insured (e.g. you get "elective" surgery on the medical advice of your doctor to avoid a bigger problem later), I don't believe those are covered by the out-of-pocket max.
The out of pocket maximum also applies to non-grandfathered group insurance plans. There were some plans allowed to be grandfathered, so the plans wouldn't terminate as non-compliant, but in practice the vast majority of plans terminate every year, and essentially everyone with health insurance now has an $8,500 per individual annual out of pocket maximum. I'm sure if you look at your own insurance, you'll find $8,500 or less.
It's true this cap only applies to in-network, approved care. But that's the same under any health plan, whether universal, or not. For instance, Sovaldi and Harvoni are curative for hepatitis C, but it costs $50,000 for a 12-week course of treatment in the UK. There are 210,000 people with hepatitis in the UK, but the NHS only furnishes 10,000 courses of treatment per year. If you're not approved, you can't get it from the NHS, but you're free to buy it yourself, of course.
