It's alive, kickin', and has had 3 Pull Reqs so far.
Ack.vim's author 👎'd the original patch I did to make it work with ag, so we mutually decided it was better to split out this into its own thing. It's pretty nice to be able to :Ag, IMO.
(I didn't make this very clear: My idea is to link to that instead of Ack.vim)
I already link to https://github.com/rking/ag.vim in the readme. Should I change the link to the vim-scripts repo instead?