Command turris-diagnostics is not present on older version of Turris OS
Imagine what situation which I dealt with in our support department. You bought the router, and it had a factory version. It could be 4.0.1 and Foris does not work. The user found encouragement to SSH to his router, and the script is not there.
We should somehow mention our documentation or provide more ways to get diagnostics from SSH. @srechner would you like to write a few rows to our documentation about it?