This was apparently hosted on the long gone "apollo" server[1], and when
archweb was migrated to a dedicated cloud VM, it was changed to a
redirect to the main site (archlinux.org)[2][3].

It may have made sense at the time, but now four years later there is no
reason for keeping this around.

I guess dev.archlinux.org was something similar to what pkgbuild.com is
today ("Public HTML server" for staff), but only for developers.

[1] f6c3af0e ("Merge branch 'apollo_decomission' into 'master'")
[2] 824fb084 ("tf-stage1/archlinux: Change DNS records for the archweb migration and also increase the machine size")
[3] 9800d023 ("roles/archweb: Create domain redirects for the domains that point to specific archweb sub urls.")