Comment 540 for bug 1218322

Kim,

First try this:

Install gnome-tweak-tool (should pull in gnome-tweaks)

Than in the gui:

Tweak tool >> Typing >> Modifiers of input sources switch >> Alt shift.

First time, I was able to bypass this bug by installing Unity desktop (planned on it anyway), and configuring the shortcut there. The settings are transferred to gnome shell settings.

how-to install unity has been detailed here:
https://linuxconfig.org/ubuntu-20-04-unity-desktop

In case you want to keep it:
https://discourse.ubuntu.com/t/testing-unity-session-in-focal-fossa-20-04/13692

And how to remove it:
https://www.makeuseof.com/tag/remove-unity-desktop-ubuntu/

But my guess is that in gnome shell it was disabled intentionally as it breaks other shortcuts:
https://askubuntu.com/questions/1075604/disabling-altshift-to-switch-keyboard-layouts-in-ubuntu-18-04
https://unix.stackexchange.com/questions/333368/gnome-3-22-disable-altshift-keyboard-layout-switching

There are also some suggestions for command line config, but I did not try this:
https://unix.stackexchange.com/questions/316998/how-to-change-keyboard-layout-in-gnome-3-from-command-line