The idea would be to trust private IP ranges as trusted proxies by default. This would make Symfony works by default when using it behind a load balancer running on the same local network (leaving explicit configuration needed for cases where you have a trusted proxy on public internet, for instance when using Cloudflare without their Argo Tunnel feature).
For instance, this would work out of the box on Heroku (I haven't investigated AWS ELB as I'm not using it). It would also remove the need for HttpCache to alter the trusted proxies to force 127.0.0.1 to be part of it

For reference, this is what Rails does: https://api.rubyonrails.org/v5.1.7/classes/ActionDispatch/RemoteIp.html

what do you think about it ?