Q&A forum for developers.
StackExchange API is a free Development API that requires OAuth authentication, is served over HTTPS and has unconfirmed CORS support.
Details from api.stackexchange.com.

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The StackExchange API is provided by Stack Exchange (api.stackexchange.com).
Yes. The StackExchange API is listed in the Development 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.
Yes. The StackExchange API uses OAuth authentication, so you need to register for credentials before making requests.
CORS support for the StackExchange API is unknown. Test a request from the browser, and fall back to a server-side call if it is blocked.
Yes. The StackExchange API is served over HTTPS, so it's safe to use in production.