RATP Open Data API.
Transport for Paris, France API is a free Transportation API that requires no API key, is only available over HTTP and has unconfirmed CORS support.
Portail Open Data RATP
Details from data.ratp.fr.

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The Transport for Paris, France API is provided by API — Open Data RATP (data.ratp.fr). Portail Open Data RATP
Yes. The Transport for Paris, France API is listed in the Transportation 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 Transport for Paris, France API requires no authentication, so you can start making requests without signing up.
CORS support for the Transport for Paris, France API is unknown. Test a request from the browser, and fall back to a server-side call if it is blocked.
No. The Transport for Paris, France API is only available over plain HTTP, which browsers block on HTTPS sites. Call it from a server instead.