Slovenia Government Open Data.
Open Government, Slovenia API is a free Government API that requires no API key, is served over HTTPS and does not support CORS, so it should be called from a server.
Details from podatki.gov.si.

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The Open Government, Slovenia API is provided by Odprti podatki Slovenije (podatki.gov.si).
Yes. The Open Government, Slovenia 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, Slovenia API requires no authentication, so you can start making requests without signing up.
Not directly. The Open Government, Slovenia API does not allow CORS, so call it from a server, serverless function or proxy instead of front-end JavaScript.
Yes. The Open Government, Slovenia API is served over HTTPS, so it's safe to use in production.