For a few years now, libinput has provided button scrolling. Holding a designated button down and moving the device up/down or left/right creates the matching scroll events. We enable this behaviour by default on some devices (e.g. trackpoints) but it's available on mice and some other devices. Users can change the button that triggers it, e.g. assign it to the right button. There are of course a couple of special corner cases to make sure you can still click that button normally but as I said, all this has been available for quite some time now.
New in libinput 1.15 is the button lock feature. The button lock removes the need to hold the button down while scrolling. When the button lock is enabled, a single button click (i.e. press and release) of that button holds that button logically down for scrolling and any subsequent movement by the device is translated to scroll events. A second button click releases that button lock and the device goes back to normal movement. That's basically it, though there are some extra checks to make sure the button can still be used for normal clicking (you will need to double-click for a single logical click now though).
This is primarily an accessibility feature and is likely to find it's way into the GUI tools under the accessibility headers.