GNOME 3.28 uses clickfinger behaviour by default on touchpads

To reduce the number of bugs filed against libinput consider this a PSA: as of GNOME 3.28, the default click method on touchpads is the 'clickfinger' method (see the libinput documentation, it even has pictures). In short, rather than having a separate left/right button area on the bottom edge of the touchpad, right or middle clicks are now triggered by clicking with 2 or 3 fingers on the touchpad. This is the method macOS has been using for a decade or so.

Prior to 3.28, GNOME used the libinput defaults which vary depending on the hardware (e.g. mac touchpads default to clickfinger, most other touchpads usually button areas). So if you notice that the right button area disappeared after the 3.28 update, either start using clickfinger or reset using the gnome-tweak-tool. There are gsettings commands that achieve the same thing if gnome-tweak-tool is not an option:

$ gsettings range org.gnome.desktop.peripherals.touchpad click-method             
enum
'default'
'none'
'areas'
'fingers'
$ gsettings get org.gnome.desktop.peripherals.touchpad click-method
'fingers'
$ gsettings set org.gnome.desktop.peripherals.touchpad click-method 'areas'

For reference, the upstream commit is in gsettings-desktop-schemas.

Note that this only affects so-called ClickPads, touchpads where the entire touchpad is a button. Touchpads with separate physical buttons in front of the touchpad are not affected by any of this.