FabioLolix [1] filed a deletion request for mono-beta [2]: Last updated on 2018-02-19, beta channel is no longer present on https://www.mono-project.com/download (Nightly, Preview, Stable, Visual Studio are available)
[1] https://aur.archlinux.org/account/FabioLolix/ [2] https://aur.archlinux.org/pkgbase/mono-beta/
