SSH -X "Warning: untrusted X11 forwarding setup failed: xauth key data not generated"

Note that some incomplete answers might lead to security flaws.

1. Using ssh -Y means here having fake xauth information which is bad!
2. ssh -X should work since XQuartz, once enabled, uses xauth. The only problem is that ssh is looking for xauth in /usr/X11R6/bin and on macOS with XQuartz it is in /opt/X11/bin

Secure solution:

1. Enable the first option in the Security tab of preferences (Cmd-,) which enables authenticated connections.

2. Edit ~/.ssh/config, add XAuthLocation /opt/X11/bin/xauth to the host config.

3. ssh -X your_server works in a secure manner.

4. Ensure xauth is installed on the destination host.

On macOS Sierra, I now have to do ssh -Y instead of ssh -X to get a display from a linux machine to work on my Mac.