What are the detailed specifications about "Added support for automatic HTTPS upgrades." ?

In safari-15-beta-release-notes (https://developer.apple.com/documentation/Safari-Release-Notes/safari-15-beta-release-notes),

Added support for automatic HTTPS upgrades.

is written as one of new features regarding security & privacy.

I want to know more detail about this, but there is no article in webkit blog about this.

If you know about this, please tell me the detailed specifications about "Added support for automatic HTTPS upgrades."