FredBezies [1] filed a deletion request for virtualbox51 [2]: Long dead version. You can read this on https://www.virtualbox.org/wiki/Download_Old_Builds:
"VirtualBox 5.1 (no longer supported, support ended 2018/04)" [1] https://aur.archlinux.org/account/FredBezies/ [2] https://aur.archlinux.org/pkgbase/virtualbox51/
