SSH protocol version 1, the original SSH protocol, is severely flawed, and should never be used. It's still in most Linux distributions, but fortunately, it's always disabled by default. However, let's say you open your /etc/ssh/sshd_config file and see this:
Protocol 1
Alternatively, you may see this:
Protocol 1, 2
If you do, then you have a problem.
The Ubuntu man page for the sshd_config file says that protocol version 1 is still available for use with legacy devices. However, if you're still running devices that are that old, you need to start seriously thinking about doing some upgrades.
As Linux distributions get updated, you'll see SSH protocol 1 gradually being completely removed, as has happened with Red Hat and CentOS as of version 7.4.
