Googled for hours, until eventually found the solution. Posting this to help others.
After re-installing Windows 10 from scratch, my touchscreen pinch-to-zoom does not work. No zoom happens by touch, only by CTRL + and CTRL - on the keyboard.
PC is a Lenovo Thinkpad X230 Tablet PC with a Touchscreen. It supports multi-touch, and Windows worked with "pinch zoom" before this re-install.
Lots of articles and Superuser.com help says go to Control Panel -> Mouse -> Additional Mouse Options -> Device Settings, but after installing the Lenovo Thinkpad touchpad drivers, the Device Settings is replaced with a Thinkpad tab. Only options available are Touchpad settings, including pinch-and-zoom on the touchpad (not touch screen). There is no way to enable or disable the touch-screen's pinch-to-zoom feature.
Maybe this issue was caused by the order I installed the drivers?