Centos – “Created slice user-0.slice.” and “Starting Session 2025 of user root.” in /var/log/messages

Sure, edit /etc/syslog.conf (or /etc/rsyslog.conf depending on distro). There's a line

*.=info;*.=notice;*.=warning;\
    auth,authpriv.none;\
    cron,daemon.none;\
    mail,news.none      -/var/log/messages

remove *.=notice or *.=info or tweak it as you want.

$ man syslog.conf

will give you all the options you can use.

Don't forget to restart syslog daemon for changes to take effect.

You’re not specifying the Type. By default, it is simple, which expects that the service process not exit.

According to the project page, running in the foreground is as simple as not passing -d start.

The complete unit file would look like this:

[Unit]
Description=Shadowsocks-R Server
After=network.target

[Service]
ExecStart=/usr/local/shadowsocks/server.py -c /etc/shadowsocks-r/config.json

[Install]
WantedBy=multi-user.target

Systemd will automatically take care of monitoring the process, restarting and stopping it as requested/required.