Account aging!
/sourceware/infra/bin/list-ssh-login produces a report of every gid=1000 user who has or hasn't logged on, as per given /var/log/secure* log files (containing ssh authentication records)
/sourceware/infra/bin/list-ssh-login-email mass-emails those who are passive.
/sourceware/infra/bin/user-retire retires a single given user
NB: the above scripts deal with access at the individual account level, not the shared-account per-key level.