I previously used Fn+F9 to disable/enable my touchpad for when I used an external mouse (when using Ubuntu 16.04). After upgrading to 18.04 I've found that when I hit that combination I get a graphic showing the touchpad icon with an "X" but it actually doesn't do anything at all.
I have an Asus UX305C
$ lsmod | grep asus
asus_nb_wmi 28672 0
asus_wmi 28672 1 asus_nb_wmi
sparse_keymap 16384 1 asus_wmi
wmi 24576 1 asus_wmi
video 45056 2 asus_wmi,i915
asus_wireless 16384 0
$ dconf read /org/gnome/desktop/peripherals/touchpad/send-events
'enabled'
EDIT: I discovered that it works correctly in the login screen and also works fine when using a LiveUSB 18.04. I think the issue is some residual package or setting from a 16.04.
