Horizontal scrolling on my touchpad was disabled upon upgrading to 17.10 and I can't find a way to re-enable it in the settings.
Two finger (horizontal and vertical) scrolling was the default behavior when I was using 16.10, even after I had switched to the GNOME desktop. However, there seems to be no settings for this both in the settings menu and dconf that I can find.
What settings app are you using?
– Calixte Nov 08 '17 at 15:24