Feel free to mark this as writable. I agree that using this file for system-wide proxy makes a ton of sense-- if we only put it in the snapd service files, then we’ll just get another request to allow managing /etc/environment for everything else. I’m not sure if there are use cases for having both /etc/environment editable and having snapd use a different proxy, but that would be an easy enough change in snapd to ignore environ if configured to use something else via ‘snap set core …’ or similar.
The security policy for snaps currently does not allow writing /etc/environment. It being readonly now doesn’t provide a ton of security protection: only root can edit it, and if you are root on the system (including in a devmode snap) you could adjust systemd units for proxy, perform a bind mount on /etc/environment, etc.