The slow query log file specified by slow_query_log_file must be created in advance. Is this common knowledge?
System Variables Related to Slow Query Log
[mysqld]
slow_query_log=ON
slow_query_log_file = /var/log/slow.log
long_query_time=0
If the file is not created in advance, an error will be output.
mysqld: File '/var/log/slow.log' not found (Errcode: 13 - Permission denied)
2019-07-01T11:16:23.095556Z 0 [ERROR] Could not use /var/log/slow.log for logging (error 13 - Permission denied). Turning logging off for the server process. To turn it on again: fix the cause, then either restart the query logging by using "SET GLOBAL SLOW_QUERY_LOG=ON" or restart the MySQL server.