Manual:$wgVaryOnXFP/en

Details
Whether to add X-Forwarded-Proto to the Vary and X-Vary-Options headers for API requests and RSS/Atom feeds.

Use this if you have an SSL termination setup and need to split the cache between HTTP and HTTPS for API requests, feed requests and HTTP redirect responses in order to prevent cache pollution.

This does not affect 'normal' requests to index.php other than HTTP redirects.