In `man sshd_config`, it says `PermitRootLogin no` is the default. Therefore I left the line `#PermitRootLogin no` unchanged in /etc/ssh/sshd_config. However, I ran `ssh root@gentoo-box` just now and found it succeed with publickey authentication(challengeresponse is also ok). Then I uncomment `#PermitRootLogin no` in sshd_config and root is no longer permitted to login via ssh. My sshd_config is exacly as same as the file emerge installed default. The ebuild name is `net-misc/openssh-6.9_p1-r2`.
looks fixed in 7.0 PermitRootLogin Specifies whether root can log in using ssh(1). The argument must be “yes”, “prohibit-password”, “without-password”, “forced-commands-only”, or “no”. The default is “prohibit-password”.