Poland Government Open Data.
Open Government, Poland API is a free Government API that requires no API key, is served over HTTPS and supports CORS, so it can be called from the browser.
Details from dane.gov.pl.

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The Open Government, Poland API is provided by Otwarte Dane (dane.gov.pl).
Yes. The Open Government, Poland API is listed in the Government category of the public-apis directory, which only includes APIs that are free or have a free tier. Check its documentation for rate limits and paid plans.
No. The Open Government, Poland API requires no authentication, so you can start making requests without signing up.
Yes. The Open Government, Poland API supports CORS, so front-end JavaScript can call it directly.
Yes. The Open Government, Poland API is served over HTTPS, so it's safe to use in production.