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