libinput 1.1.5 has a change in how we deal with semi-mt touchpads, in particular: interpretation of touch points will cease and we will rely on the single touch position and the BTN_TOOL_* flags instead to detect multi-finger interaction. For most of you this will have little effect, even if you have a semi-mt touchpad. As a reminder: semi-mt touchpads are those that can detect the bounding box of two-finger interactions but cannot identify which finger is which. This provides some ambiguity, a pair of touch points at x1/y1 and x2/y2 could be a physical pair of touches at x1/y2 and x2/y1. More importantly, we found issues with semi-mt touchpads that go beyond the ambiguity and reduce the usability of the touchpoints.
Some devices have an extremely low resolution when two-fingers are down (see Bug 91135), the data is little better than garbage. We have had 2-finger scrolling disabled on these touchpads since before libinput 1.0. More recently, Bug 93583 showed that some semi-mt touchpads do not assign the finger positions for some fingers, especially when three fingers are down. This results in touches defaulting to position 0/0 which triggers palm detection or results in scroll jumps, neither of which are helpful. Other semi-mt touchpads assign a straightforward 0/0 as position data and don't update until several events later (see Red Hat Bug 1295073). libinput is not particularly suited to handle this, and even if it did, the touchpad's reaction to a three-finger tap would be noticeably delayed.
In light of these problems, and since these affect all three big semi-mt touchpad manufacturers we decided to drop back and handle semi-mt touchpads as single-finger touchpads with extra finger capability. This means we track only one touchpoint but detect two- and three-finger interactions. Two-finger scrolling is still possible and so is two- and three-finger tapping or the clickfinger behaviour. What isn't possible anymore are pinch gestures and some of the built-in palm detection is deactivated. As mentioned above, this is unlikely to affect you too much, but if you're wondering why gestures don't work on your semi-mt device: the data is garbage.