I don’t think it was ever a package in EndeavourOS repository, but an app in the eos-bash-shared package.
EDIT: checked it in the git history. eos-shifttime was added into package eos-bash-shared
at 2021-Jul-21. First it was named very briefly (about 8 hours 
) as shifttime, then renamed to the current name.
And the app was originally written by @freebird54 who generously admitted it into our repo.