Neovim is in community repository, and visible on my system. Do you have community repository enabled?
I'm using: (/etc/pacman.d/mirrorlist)
## Europe
#Server = https://mirrors.dotsrc.org/artix-linux/repos/$repo/os/$arch
#Server = http://ftp.ntua.gr/pub/linux/artix-linux/$repo/os/$arch
#Server = https://ftp.cc.uoc.gr/mirrors/linux/artixlinux/$repo/os/$arch
Server = https://quantum-mirror.hu/mirrors/pub/artix-linux/$repo/os/$arch
# ...etc...
(/etc/pacman.d/mirrorlist-arch)
## Worldwide
Server = https://mirror.f4st.host/archlinux/$repo/os/$arch
Server = https://mirror.selfnet.de/archlinux/$repo/os/$arch
# ...etc...
(/etc/pacman.conf)
[community]
Include = /etc/pacman.d/mirrorlist-arch