I have something similar, but open fzf to select the branches to delete [1].

    function fcleanb -d "fzf git select branches to delete where the upstream has disappeared"
        set -l branches_to_delete (
            git for-each-ref --sort=committerdate --format='%(refname:lstrip=2) %(upstream:track)' refs/heads/ | \
            egrep '\[gone\]$' | grep -v "master" | \
            awk '{print $1}' | $_FZF_BINARY --multi --exit-0 \
        )

        for branch in $branches_to_delete
            git branch -D "$branch"
        end
    end