If ssh dsa key is used for password-less access to other boxes, it won’t work in MacOS Sierra anymore. It’s because:
The problem is that DSA keys are obsolete after OpenSSH 7.0, which the new system seems to use. I generated a new RSA key and things worked again. Also, in the process I found out that the authorized_keys2 file has been actually deprecated for a long time! So I advice everybody to use authorized_keys instead, which works both with protocols 1 and 2. That way Apple’s original sshd_config will work as is.
So be sure to redo your key in rsa before (or after) the OS upgrade.