I didn't knew this until Ubuntu Budgie announced its release notes. It's seems that Ubuntu Budgie uses this instead of mutter. See I learned something new 
It contains functionality related to, among other things, window management, window compositing, focus tracking, workspace management, keybindings and monitor configuration.
Internally it uses a fork of Cogl, a hardware acceleration abstraction library used to simplify usage of OpenGL pipelines, as well as a fork of Clutter, a scene graph and user interface toolkit.
Magpie is a soft-fork of GNOME's mutter at version 43 tailored to the requirements of the Budgie Desktop 10 series (from v10.8 and later). Magpie allows mutter based desktops such as GNOME Shell to co-exist since the key-components such as libmagpie are separated by both name and file-system install location. Magpie shares some Mutter shared files; therefore these need to be delivered/installed as part of the distribution from its mutter package. Alternatively these files can be delivered using the meson option "with_shared_components" where budgie-desktop is not required to co-exist with any mutter based desktops.