MailChimp HQ has been running on something akin to the GitHub flow for a while now. Side effect of this strategy is there are So. Many. Branches.
Even with regular cleanup via git branch -d branchName or the harsher git branch -D branchName, references to other remote branches linger. If your coworkers are good about clearing out their remote branches, it’s easy make your local copy shiny again.
git remote prune origin will clear out local references if the remote branches no longer exists. Appending it with --dry-run will let you see what it’ll do.
If you want to do this for all of your remotes at once git fetch -p will do the trick.
Other resources: