Interesting. That is a separate experimental low-data setting that's been around for a while in Chrome, but isn't supported by Safari [0]. Haven't run across it before, thanks!

For low data mode specifically, looks like you can easily get "allowsConstrainedNetworkAccess" from an iOS app [1], but don't see anything comparable available for Safari or Firefox.

[0] https://caniuse.com/#search=save-data [1] https://developer.apple.com/videos/play/wwdc2019/712/

Edit: typo