Log entries are purged from storage according to elaborate rules explained here, for different types of log entry. How to extend time coverage in those logs, and how to get free statistics about them.
logd
An introduction to the Unified log used on Macs and Apple’s devices. Its goals, how logs are stored and maintained, their content, privacy protection, and tools available to access them.
Introduced with Mac OS X, logs became a good way to diagnose a Mac’s problems, using Console. Then in 2016 it all changed, and not for the benefit of administrators or users.
The macOS log is very different from those seen in Unix. This explains where and what log files are, how they get written, how much they store, and how quickly entries can be written.
A new Frequency view shows total log entries from different subsystems, helping you mute those that are clogging up your log.
Does your Mac’s log go back far enough to be useful, or is it full of junk? Here’s how to control what takes up space, and a new version of Mints to help.
Checking your Mac’s log assumes that the records are still there. What happens when they only last 24 hours, and you’re looking for anti-malware scans run once a day?
Sometimes, when you return to examine entries in the log a few hours after they were written, many of them have vanished. How come?
Found in some supplementary log files in Sierra only, and contain a UUID which may refer to main log files.
Found in supplementary log files in Sierra and High Sierra only. May contain a UUID reference to main log files.
The Eclectic Light Company 