How to use window menu accelerators/mnemonics in Ubuntu 18.04 LTS?

On GNOME Shell it is impossible! Welcome to the new brave world with unpredictable limits and problems!

To get these shortcuts on Ubuntu 18.04 LTS and 20.04 LTS in place you can switch to:

• Unity - installable with sudo apt-get install ubuntu-unity-desktop:

  

• GNOME FlashBack - installable with sudo apt-get install gnome-panel:

  

• MATE Desktop Environment (my favorite, as it avoids GNOMEs Hell) - installable with sudo apt-get install ubuntu-mate-desktop^:

  

• XFCE (xubuntu-desktop^) does not have mnemonic for Always on Top, but usable and looks very nice:

  

• Cinnamon (cinnamon-desktop-environment) have all necessary mnemonics:

  

• KDE (kubuntu-desktop^) have mnemonics, they may be accessed from Alt+F3 (so called Window Operations Menu) - for example to keep window above others you need to click Alt+F3, M (for More Actions), A (for Keep Above Others) and menu is rich:

  

• Openbox (openbox), LXDE / LXQT / Lubuntu (lubuntu-desktop^, lxqt) have mnemonics, they are accessed with Alt+Space:

  

• Budgie (ubuntu-budgie-desktop^) have mnemonics, they are accessed with Alt+Space:

  

Unfortunately, there is no mnemonic support anymore for the Alt+Space window menu. There appears to be no intentions to bring it back. The current bug report is two years old. The original issue was filed back in 2014.

Still, keyboard shortcuts are available for the most common actions. These are quicker as they do not require you to see the menu and reduce the effort to a single keystroke.

• Minimize: Super+h.
• Maximize: Super+↑.
• Restore: Super+↓. Alt+F10 toggles between maximize and restore.
• Move: Alt+F7.
• Resize: Alt+F8.
• Close window: Alt+F4.

These key assignments can be changed in Settings - Keyboard.

For Always on top, no keyboard combination is defined by default, nor exposed through Settings - Keyboard. You still can define a key combination using dconf-editor, or a gsettings command. For example, to set Super+t for toggling a window on top, issue the command

gsettings set org.gnome.desktop.wm.keybindings always-on-top "['<Super>t']"

Undo with the command

gsettings reset org.gnome.desktop.wm.keybindings always-on-top

The KISS (Keep It Simply Stupid) answer to that is to do a simple:

Alt+space and then ↓↓↓↓↓Enter

not as good as Alt+space and then i, but still better than:

• lifting your hand from the keyboard,
• looking for the mouse,
• pointing,
• clicking
• moving your hand back to the keyboard...

¯\_(ツ)_/¯

Actually, there are several shortcuts to do so.
I don't know what they are in earlier ubuntu.
They are my windows behaviours.
Windows / action / ubuntu
Alt+Space & n minimize Super+h
Alt+Space & x maximize Super+↑
Alt+Space & r unmaximizeSuper+↓
Alt+Space & c close Alt+F4
Alt+Space & m move Alt+F7
Alt+Space & s resize Alt+F8
There are more in

setting  >  device  >  keyboard

You can define or redefine all these shortcuts there.
But I thought pressing three keys......It's your choice.

The GNOME Shell extension Menu Keyboard Shortcut by Edward Jiang fixes my most used maximize/minimze mnemonic.