In my test environment with Python script I generate on a target server:
SSHException: Error reading SSH protocol banner
simply opening/closing a sufficient number of ssh connection.
Is it classified as a DOS attack against sshd?
Maybe is not classified as DoS attack but I think yes, this could cause DoS if you are able to generate enough traffic. To protect this you can use something like fail2ban
I would say, as long you are not killing SSH daemon it is a stress test. Check the following settings in sshd config: MaxSessions and MaxStartups.