Comment 2 for bug 2038416

The "juju-" prefixed proxy declarations are set in the environment for hook contexts, so that the charm may use them at its discretion.

The ones you refer to as legacy are the ones that are set in the machine's /etc/profile.d and apply to all requests.

The Charmhub client is not running in a hook, and so requires the latter to be set in order to proxy correctly.

If you don't want to set it for all models, I think you can pass it simple as config to bootstrap rather than model-defaults.