Part of macOS
logdSystem log
○ Leave it
Keeps the system log that apps and macOS write to.
What it is
Collects log messages for troubleshooting.
If you stop it
macOS starts it again straight away. Leave it.
The verdict
○ Leave it. Part of macOS, or something that starts again by itself. You never need to close these.
In Activity Monitor
logdlogd_helpersyslogddiagnosticd
From Procession’s guide, in the words the app shows.