Minimize window keyboard shortcut... impossible?
Solution 1:
I don't have a minimize window in Ubuntu 18 either. Perhaps you mean "Hide window" for which the default mapping is Super-H. However, the button in the toolbar is still called minimize.
Solution 2:
Try Super+H (H standing for "hide").
There are lots of keyboard shortcuts to navigate the UI without a pesky mouse or touchpad getting in the way. 👍🏻