Merely pressing Super alone and releasing it will show all your Windows now. In addition, also the keyboard shortcuts Super+s and Alt+F1 will show the window overview.
Changing the shortcut combinations to show the window spread
If you wish, you can set it to Super+w by changing the assignment of "Show the overview" in Settings - Keyboard. That, however, will replace the second shortcut Alt+F1 and leave the Super+s shortcut in place. If you want full control on what key combination or combinations are assigned to that function, you can change the dconf setting
org.gnome.desktop.wm.keybindings. For example, to set both Super+w and Alt+F1, you could change the key using
dconf-config-editor or with a terminal command:
gsettings set org.gnome.desktop.wm.keybindings panel-main-menu ['<Super>w','<Alt>F1']
To reset to default:
gsettings reset org.gnome.desktop.wm.keybindings
As to why the change?
Ubuntu used to have its own in-house developed desktop, Unity. With version 17.10, they moved to the Gnome Shell desktop, where Super alone, Super+s and Alt+F1. So with respect to these hotkeys, they decided to adhere to the conventions used in Gnome Shell rather than trying to continue their previous choices in the Unity desktop.