How to automatically switch ssh config based on local subnet?

You can use Match's exec option to execute shell commands, so you can write something like this:

Match host web exec "hostname -I | grep -qF 10.10.11."
    ForwardAgent yes
    ProxyCommand ssh -p 110 -q relay.example.com nc %h %p
Host web
    HostName web.example.com
    Port 1111

The Match option boolean logic can short-circuit, so put host first to skip the exec term for other hosts. Try ssh web -vvv to see the Match logic in action.