ALT+Tab no longer switches applications since upgrade to 18.04

In the Settings > Devices > Keyboard menu, you will find a list of keyboard shortcuts. Under the Navigation section, you can find the relevant functions and their associated shortcuts, and change them as you see fit. You're looking for the one named Switch applications.

Run gnome-control-center aka the Settings application.

Remove the shortcut for Switch applications by clicking on it and deleting it with the backspace key, now press the Set button.

Now set Switch windows by clicking on it, then press Alt + Tab and click the Set button.

Turns out the configuration just needs to be updated in settings.

Open Settings > Devices > Keyboard and under Keyboard Shortcuts select Switch applications and update it with left Alt+Tab.

Both Alt+Tab combinations should now work.