Nitpick, but it is actually part of the kernel. X server also uses it, but there was no problem because nvidia could release a binary patch for it to use their proprietary thingy. It is not possible with the many wayland compositor so it became an apparent problem with wayland.
“There are many distinct software components in desktop ecosystem. There are tools like Mesa for rendering (and each of its drivers), the Linux KMS/DRM subsystem, buffer allocation with GBM, the userspace libdrm library, libinput and evdev, and much more still.
[...]
libinput
Like libdrm abstracts the DRM subsystem, libinput provides the userspace end of evdev.”
