just to clarify AUR stands as Arch Linux User Repository and is not related or supported officially by Arch Linux.
more plain, Arch don't support Unity or Mir at all and never did, there are just User Made packages for it in AUR.
quote from faq https://wiki.archlinux.org/index.php..._is_the_AUR.3F
"The AUR (Arch User Repository) is a place where the Arch Linux community can upload PKGBUILDs of applications, libraries, etc., and share them with the entire community. Fellow users can then vote for their favorites to be moved into the [community] repository to be shared with Arch Linux users in binary form."
"The Arch User Repository is where all PKGBUILDs that users submit are stored, and must be built manually with makepkg. When PKGBUILDs receive enough community interest and the support of a TU, they are moved into the [community] repository (maintained by the TUs), where the binary packages can be installed with pacman."
"A TU (Trusted User) is a person who is chosen to oversee AUR and the [community] repository. They are the ones who maintain popular PKGBUILDs in [community], and overall keep the AUR running."