Ubuntu in the past had that option, I'm not sure why they left it out.
But I can achieve that result by adding this command:
gsettings set org.gnome.desktop.peripherals.touchpad send-events 'disabled-on-external-mouse'
However, it would be interesting for people to have that via the GUI
If you like a GUI to do this you can install dconf-editor
sudo apt install dconf-editor
and go to
org > gnome > desktop > peripherals > touchpad
and enable/disable it there.
(images from ostechnix. com)
I agree, Ubuntu removing features, will not win favor awards. At least however, you can set it in the Dconf editor tool. Not every notebook, has good palm rejection, we learn this from watching LTT reviews. It boggles my mind, why this is not a feature, in the mouse & keyboard settings options. Ubuntu are not winning favors here.
