The issue file, also found in the /etc directory, shows a message on the local terminal, just above the login prompt. A default issue file would just contain macro code that would show information about the machine. Here's an example from an Ubuntu machine:

Ubuntu 18.04 LTS 
 l

Or, on a Red Hat/CentOS machine, it would look like this:

S
Kernel 
 on an m

On an Ubuntu machine, the banner would look something like this:

On a CentOS machine, it would look something like this:

You could put a security message in the issue file, and it would show up after a reboot:

In reality, is there really any point in placing a security message in the issue file? If your servers are properly locked away in a server room with controlled access, then probably not. For desktop machines that are out in the open, this would be more useful.