Privacy options are same for feeds and stacks because stacks are created as feeds and have all properties as of feeds and easily customizable but not re-named.
Privacy can be changed right at the time of sharing feed or after sharing from feeds option bar click on more options and change to Public or Private as per your convenience.
Privacy can be changed right at the time of sharing feed or after sharing from feeds option bar click on more options and change to Public or Private as per your convenience.