Computer Configuration > Administrative Templates > Microsoft Edge
Both
HKLM\Software\Policies\Microsoft\Edge
OBSOLETE: This policy is obsolete and doesn't work after Microsoft Edge 84. This policy doesn't work because it was only intended to be a short-term mechanism to give enterprises more time to update their web content if it was found to be incompatible with stricter mixed content treatment. This policy controls the treatment for mixed content (HTTP content in HTTPS sites) in the browser. If you set this policy to true or not set, audio and video mixed content will be automatically upgraded to HTTPS (that is, the URL will be rewritten as HTTPS, without a fallback if the resource isn't available over HTTPS) and a 'Not Secure' warning will be shown in the URL bar for image mixed content. If you set the policy to false, auto upgrades will be disabled for audio and video, and no warning will be shown for images. This policy does not affect other types of mixed content other than audio, video, and images.