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Steam Web API documentation.
Steam API is a free Games & Comics API that requires apiKey authentication, is served over HTTPS and does not support CORS, so it should be called from a server.
The most complete Steam Web API reference with 200+ interfaces, including undocumented APIs. Execute requests directly in your browser.
Details from steamapi.xpaw.me.

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Yes. The Steam API is listed in the Games & Comics 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 Steam API uses apiKey authentication, so you need to register for credentials before making requests.
Not directly. The Steam API does not allow CORS, so call it from a server, serverless function or proxy instead of front-end JavaScript.
Yes. The Steam API is served over HTTPS, so it's safe to use in production.