This report is just for documentation, and to get a possibly-related model number to show up in web searches.
This problem may also affect the Acer ES1-511 laptop. It is similar to the E5-511 mentioned here, but with a slower CPU and a smaller hard disk. I have an ES1-511, and the keyboard and touchpad stopped working under X. Even though I had an older version of libinput (1.12.6), which did not understand the ModelTabletModeSwitchUnreliable quirk, the change suggested in this bug *partially* fixed it; the keyboard started working but the touchpad did not. What fixed it for good for me was blacklisting the "Intel Virtual Button driver" in udev. This made both the keyboard and touchpad work under X. I posted to debian-user https://lists.debian.org/debian-user/2020/10/msg00530.html and someone there linked me to this post on the Arch user forum https://bbs.archlinux.org/viewtopic.php?pid=1925437#p1925437 which details the udev fix I used. Thanks! Matt Roberds