Google already does it with Gmail[1][2], so it's not unheard of. But it adds another layer of complexity, plus is somewhat antithetical to the privacy viewpoint as it then exposes all of the images to the app developer now since it's routed through the proxy server instead of direct end-user -> image server requests.
[1] Can't find a page specifically detailing it, but [2] gives a basic synopsis on it in the context of allowing GSuite admins to whitelist internal domains from routing through the proxy.
> it then exposes all of the images to the _app developer_
Not if the app developer proxies the encrypted traffic.
That way the app dev does not know the content and the third party does not know which original IP requested it.
This is akin to the Signal-Giphy implementation.
Developing such a tool might be valuable for privacy-conscious application developers.