mev

??

I don't remember being asked for proof of health coverage in ~35 countries I've toured[*]. There were sometimes more specific requirements such as a yellow fever certificate. Also, there was an expectation that you were not necessarily covered under their health system and would need to pay out of pocket. In Quito Ecuador, I did pay out of pocket for treatment on some saddle sores. The first medical place I found wouldn't take myself as a foreigner but they directed me to a place that would. The out of pocket price was considerably less than I would have paid in the US. I had some emergency dental procedure done in Addis Ababa, Ethiopia also out of pocket and also considerably less than in the US.

Now in some of those cases I did buy travel insurance mostly to cover extra risks, though never made a claim. This coverage also included a medical evacuation rider. When I cycled with a Dutch cyclist across Russia, her national health plan in the Netherlands also did not cover Russia, but she was able to buy a rider from a government source that extended overseas coverage as well.

What I think makes medical insurance in the US particularly important is some of the costs - which can become prohibitive if you have to pay out of pocket.

I agree that lack of (affordable) coverage can be a barrier to touring - though for some folks without that coverage it can also be an issue if they bicycle in their town, staying in the same place.
[*] It might have been on the Russian visa application, but that was long enough ago, I forget whether it was there or not.