This is a topic several of us discussed after last years SFC selection board. I don't think that individual feedback is warranted for all who board, that's the point of leadership reviewing records before and after. What I would love to see is a quantified placement for those who were not selected. For example if there were 350 reviewed for SFC in a particular MOS, but only 46 were selected for promotion, then the other 301 should get an email with a numerical ranking. If a Soldier is # 3 of 350, it can be assumed they are doing the right thing, and their record looked good, it just wasn't the best, so they missed selection due to limited slots available. If a Soldier is 340 of 350, it can be assumed they are doing something wrong, their record is messed up, or something else. Either way, the second Soldier should receive additional time with a senior mentor to determine the best way to correct deficiencies, or to discuss post mililtary options.
These days, more and more people will not be selected for promotion at all levels, and having some sort of feedback would help people prepare better for their future, in or out of the service.