Where are the postfix log files?

They should be in /var/log/mail.log. However, it appears that your syslog facility configuration was changed by you (or your provider). Using the plain syslogd instead of rsyslogd is very much possible, but then you can expect things to work different to your expectation.

As already mentioned in the comments, by installing rsyslog, the files reappeared.

On Ubuntu 16.04, I found postfix logs in /var/log/syslog:

grep postfix /var/log/syslog

You can view the stream of postfix logs:

tail -f /var/log/syslog | grep postfix