It's effectively non-required when used in patroni/postgresql/config.py. This breaks most configuration files in the wild, starting from the examples:
$ patroni --validate-config postgres0.yml
restapi.connect_address 127.0.0.1:8008 didn't pass validation: 'must not contain "127.0.0.1", "0.0.0.0", "*", "::1", "localhost"'
etcd.host 127.0.0.1:2379 didn't pass validation: '127.0.0.1:2379 is not reachable'
postgresql.connect_address 127.0.0.1:5432 didn't pass validation: 'must not contain "127.0.0.1", "0.0.0.0", "*", "::1", "localhost"'
postgresql.proxy_address is not defined.