To suppress these log entries in /var/log/messages, create a discard filter with rsyslog, e.g., run the following command:
# echo 'if $programname == "systemd" and ($msg contains "Starting Session" or $msg contains "Started Session" or $msg contains "Created slice" or $msg contains "Starting user-" or $msg contains "Starting User Slice of" or $msg contains "Removed session" or $msg contains "Removed slice User Slice of" or $msg contains "Stopping User Slice of") then stop' >/etc/rsyslog.d/ignore-systemd-session-slice.conf
Then restart the rsyslog service
# systemctl restart rsyslog
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