Hello!
According to the VyOS Wiki and the file itself, the content of vyatta-postconfig-bootup.script should be applied after the VyOS configuration is fully applied. As this is not the case anymore, many modifications via this file are not possible/working because they are applied too early. You can find an example in T209, despite the fact that the iptables chain in this example does not exist. It would be great if rc.local and therefore vyatta-postconfig-bootup.script would be executed after the configuration is fully applied, so that custom modifications can be used again.