Sometimes I think that situations in which a child would really benefit from redshirting are few and far between. If the typical K class is supposedly taught to the middle 90% of the curve, there's only 5% on the bottom end, just like on the top, i.e., just a few kids. Then again, the issue is really not core IQ levels but rate of development. One tricky aspect, as we all know, is that GT kids tend to be asyncronous, just one of the many reasons a GT kid might not fit well in a typical K classroom. I agree that the issue may be more with the structure of the school environment than with the kid. I don't remember anyone talking about redshirting when we were kids - it was around, as I'll discuss below, but the kids who were a different age for grade were few and far between.
My mother recently told me they had considered redshirting my brother, who was short with an Oct. birthday. But they didn't - someone tested him and said it would be a crime to make him wait. He didn't achieve a more normal height until the end of high school, so waiting to start K wouldn't have helped him anyway. My mom's point was that although it was tough for him to be the shortest boy for so many years, he developed other personality traits in ways he might not have but for his small stature, and that these traits are important for his current career (kinda like everyone has their cross to bear, but with a positive spin).
Like my brother, I was often the shortest in my class, but now he's about 10" taller than me LOL. Obviously it's different to be short and female, but throughout my life I usually have preferred that people underestimate me. I have always felt that conferred an edge, in life in general (from meeting random guys in bars to negotiating with opposing counsel). I don't think there's anything wrong with being the youngest in a class in that regard, when the intellectual background is present.
I know another boy with a July birthday in my DS's class who has highly intelligent (both teach at the college level) and involved parents struggling to read, so for him it was the right choice.
With this type of situation, there's also the possibility of an LD scenario, and lack of reading ability in K, or prior to K, does not necessarily speak to intelligence (my DD7 is a case in point). I'd hate to have a smart kid held back due to an LD, and end up stuck that way, a grade back, for twelve years. I guess in that sense I see similarities between redshirting and grade retention - I vaguely recall that research shows retention not to be very effective because it usually doesn't address the core cause of problems. Sometimes another year of development might do the trick, but it's got to be hard to predict that ahead of time.
The other shame about redshirting is that school administration doesn't seem to bat an eye at these kids being different age for grade, and yet gives GT parents such a hard time about acceleration.
just my two cents