This is not implemented in Gnome Shell, the desktop used by Ubuntu 18.04. In many cases, gnome extensions can implement desired features, but I do not know of an extension that specifically meets your desire to have a "Show Desktop" icon in the application switcher.
To quickly show the desktop, you can set up a hotkey to hide all windows at once (i.e., show desktop). In Settings - Keyboard, find "Hide all normal windows" under "Navigation" and assign it a shortcut, for example Super+d.
A hack you might try, if you think it is worth it, is to create a "Show desktop" launcher as outlined in this Askubuntu question. Pin it as a favorite to your Dock (or Dash if using pure Gnome). Then install the extension Super+Tab launcher, which is well maintained at the time of writing. This extension includes your not running favorites in the Alt+Tab switcher. It thus will also contain your launcher to minimize all windows. The catch however is that all favorites will now be in the launcher, unless you remove the other favorites from the dock/dash.