Why would it be implemented as a manual setting in the browser, rather
than a per-connection setting controlled by the OS?

It’s exactly this, a per-connection setting controlled by the OS and reflected as is by the browser:

https://github.com/tomayac/netinfo/blob/relaunch/README.md#metered-connections.

Cheers,

Tom

Thomas Steiner, PhD—Developer Advocate (https://blog.tomayac.com, https://twitter.com/tomayac)

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