I'm trying to determine when the last Postgres upgrade was made. I know my current version (through "show server_version" command, and other ways too). But I wasn't able to find a history of the last upgrades and the respective dates. Is there such a thing?
An analogy: this can be achieved with the file $II_SYSTEM/ingres/version.dat in an Ingres DBMS.
You're using Redhat, so it's stored under dnf history list. It's a function of the package manager, not the database. You can build your own PostgreSQL and install and that won't show under the package manager.
The database engine itself doesn't track revisions to itself in the database: and, it would be weird if it did. If Redhat issues an upgrade and restarts the server, the server is supposed to know that Redhat issued an upgrade? What if the upgrade was a minor fix that Redhat pushed out ahead of PostgreSQL?