Lastest macOS Catalina Beta shows the identical version to the current Mojave ssh
ProductName: Mac OS X
$ /usr/bin/ssh -V
OpenSSH_7.9p1, LibreSSL 2.7.3
Right.. I finally figured it out.
It still was the damned Checkpoint Endpoint Security VPN client. Apparently it installs a kernel extension com.checkpoint.cpfw that the service com.checkpoint.epc.service controls according to organizational policy. In my case it was dropping all incoming connections. Shutting down service doesn't flush the rules enforced ...
Open the keychain app, click on the category "passwords", click add, and put the path to the SSH file as the account name:
Now, ssh-add -A should automatically add the key to your ssh-agent without any further hassle.
A terminal connection, such as created by SSH, basically transfers characters, not keystrokes. There are some exceptions to this, such as the "return" key and CTRL+letter combinations, because they have been in use since time immemorial and have their own character codes, and some keys like the arrow keys are handled by sending multi-character sequences, but ...
I will assume that your situation is like mine and most other at home: Your Mac mini and other computer is on a local network, you have a router and is connected to the internet using a service provider. Then there are four things needed to reach you mini from the internet.
1) The mac mini has to be "opened" for access. It seems you have done this already.
I have just had the same issue and after some head-scratching it turns out to have been a mis-configured ssh configuration file. I want to limit log on authentication to ssh keys only and had updated /etc/ssh/sshd_config file, but made a mistake. I suggest you check your ssh configuration file carefully.
In my case I set some parameters with the value No (...
After I post the question, I found the answer, the following command will generate the public key using the private key file:
ssh-keygen -f -y "MyPrivateKey.pem" > "MyPublicKey.pub"
then to remove it:
ssh-add -d "MyPublicKey.pub"
Internal or external? Which (exact model) of Mac Mini? Without that info this answert is gonna be vague.
If it is an external SSD connected via USB-C/Thunderbolt. you could do what I did, install masOS on the external and boot from it and then use migration utility to move everything to the external SSD and used the old internal drive for incidental storage ...