What's the state of SSH?
I've just being promoted as the Linux sysadmin of a network where they badly want to keep the linux systems as is: some Redhat 4.2 and 6.x servers/workstations.
We are also going to use SSH for the 1st time on these old linux systems. So, can I use the old ssh software that was bundled with these old linux implementations? Will these old SSH works fine (these are SSH dating back of several years ago..).
Thanks

I'm not knowledgeable on the important differences between the version of SSH that came with Redhat 4.2 and the one in use today, but I think the bigger issue/problem is with this statement: "they badly want to keep the linux systems as is: some Redhat 4.2 and 6.x servers/workstations".

Redhat 4.2 came out in 1997 and besides changes to SSH, there have been numerous security and bug fixes as well as added utilities in the versions since then. I can't imagine any valid reason for staying with that old of a version. You may want to research the broader issues of the differences between Redhat 4.2 and todays Linux version(s) and propose to the management that you should be allowed to upgrade the systems.

Sony:

I have connected SSH on a RedHat Enterprise Linux 4 system to a Redhat 7.1 system with no changes. I suspect that if the SSH configuration is left at the default and the SSH daemon is running then it can connect defaulting to the lowest common denominator of security (probably the settings from the oldest version of OpenSSH.)

I am not sure how familiar you are with Linux. You may want to lookup the hardware/software requirements for RedHat 7.3 and or 9.0 (there were many problems with RedHat 8.0) to see if all of the systems meet those requirements. If so, you could do upgrade installs and bring all systems up to the same version. Then there would not be any SSH discrepancies. I did not mention Fedora as it is fairly new and I doubt the older systems will meet its requirements (at least if you were planning on using XWindows.)

Another, possibly less time consuming way would be to download the latest version of OpenSSH. Try a test install on one of the servers. Chances are it will abort and tell you which additional packages/libraries are required. If it is more than two or three, forget it it will probably be too much of a hassle.