Sometimes you don't have an IPv6 configured and you need to SSH to an IPv6 server, you can use a bastion server for that.

Host *
    ForwardAgent yes
VerifyHostKeyDNS no
    StrictHostKeyChecking no
    GSSAPIAuthentication no
    HashKnownHosts no
    TCPKeepAlive yes
    ServerAliveInterval 60
    ProxyCommand ssh -W %h:%p <bastion server>
    IdentityFile ~/.ssh/id_rsa

Unfortunately, it does not work with IPv6 :( I don't know why, but I found a fix using my dear old friend socat

Host pi
   ForwardAgent yes
   ProxyCommand ssh -q -A <bastion server> "~/socat STDIO TCP:[2404:XXXX:XXXX:58XX::a38]:22"
   #ProxyCommand ssh -q -A <bastion server> "~/socat STDIO TCP6:IPv6.FQDN:22"
   User root
   IdentityFile ~/.ssh/id_personal

Another cool trick to access servers behind a second bastion is to use the second bastion in ProxyCommand

Host machine.domain.casa
   ForwardAgent yes
       #ProxyCommand ssh <second bastion> nc 192.16.190.192 22
       ProxyCommand ssh -W 192.16.190.192:22 <second bastion>
   IdentityFile ~/.ssh/id_personal

Use the first, commented out, ProxyCommand line if your second bastion does not forward the credentials correctly (Dropbear, very old OpenSSH)