I have the external wireless "ThinkPad TrackPoint Keyboard II"
(https://www.lenovo.com/us/en/p/accessories-and-software/keyboards-and-mice/keyboards/4y40x49493).

With this keyboard, middle-mouse scrolling works only vertically. In the
past, with a different keyboard/mouse combo device (a wired external
ThinkPad keyboard), I've been able to scroll both horizontally and
vertically. I don't know if this is an issue with the handling of this
model, or an issue that changed in libinput.

- Josh Triplett