The Polish Embassy in the U.S. says the following:embassy website

I don't know whether they ever actually check for proof of insurance in practice unless you're applying for a visa, but I doubt it, and I know I wasn't asked for it. I had heard anecdotally a few years back of some visa waiver nationals whose first Schengen entry was into Poland being checked, but have no idea if this was true. Still, the requirement does exist.

Edited to add: Although the requirement does appear on the "Visas" page of the website, it's under "General Information," not only under the list of things required when applying for a visa. And the next item below it says " In accordance with the provisions of law, the Polish border guard has the sole authority to admit an alien into the territory of the Republic of Poland at the port of entry. This rule pertains to both aliens traveling on passports only, when no visa is required on the basis of bilateral agreements, and those to whom a visa had been issued at the Polish Consulate." So it does look like the rule applies to everyone, not just those who need a visa. But of course that is a different question than whether it's enforced.