> However OS (at least windows) has an way to set network connection as a metered so software can make informed decisions. Also Android has "Data Saver" function which should also be honored by software.

Unfortunately, that automation is unreliable. It doesn't work across operating systems - Windows laptops won't enable data-saver mode when connected to iPhones and macOS laptops won't when connected to Android phones, and neither will enable it when connected to, say, public transport wifi.

And even if the OS has the information, websites can't reliably use it either. Firefox and Safari both don't implement the NetworkInformation API [1].

[1] https://developer.mozilla.org/en-US/docs/Web/API/NetworkInfo...