Mysql could not be started because unknown variable 'log-syslog=1'
I've run into the same problem as yourself and found this question while looking for solution. Eventually I found the culprit. 

You will find that /etc/mysql/mysql.conf.d/mysqld_safe_syslog.cnf contains 2 rows:

[mysqld_safe]
syslog


The only thing you have to do is remove syslog. I just commented it out:

[mysqld_safe]
#syslog


And now it works.
It is weird that it comes preconfigured like that since it is deprecated/removed (I did a clean install of v8.0.15).