alt Hacker NewsPostgres is old but a lot of its features are new. You can't really say it's mature software, as much as it has changed over its lifetime.
Truly mature software doesn't get used, because people see low activity on github and assume it's "dead" or see no new features in years and assume that it's stagnating.
Note how Robert mentions a sharp increase in back-patched commits. Many of these bugs have indeed been around, silently, for a long time.