The OpenSSH upgraded to version of today automatically
By default nothing is updated automatically. Even if you enable automatic updates, you can check when they are installed via App Store's Updates tab. If there's nothing for today, no updates are installed today — simple.

This assumes you haven't installed or set up something non-default administrative update tool. It's perfectly possible to install non-default SSH client and then have a script to check updates for it daily and install them if available. This probably isn't the case here.

And as one can install such a script it is also possible to do so unknowingly (by something malicious)—but, again, this probably isn't the case in hand.



In this case, we're putting too much trust on the header section of the man page.

The ssh man page, located in /usr/share/man/man1/ssh.1, is missing a .TH line, where the date you referred to should be defined, so presumably it's just printing a fallback: "BSD" and current date. 

Date can also be defined with .Dd line, which in ssh.1 is .Dd $Mdocdate: September 21 2017 $, which might be incorrectly interpreted on macOS.

If you wish to know when the SSH version you are using was released (note: released, not installed), you can check the current version with command ssh -V and cross reference that info with release notes to check the release date.