Our school district absolutely won't grade-skip, I guess because they have an excellent full-time G/T program that starts in 4th grade.
I noticed on the high school website that kids in high school here can only take one college course during high school that can "count" for their GPA. Our local public school offers something like 22 AP courses, so I guess they don't want kids also taking a million university or community college course that will "count" to raise their GPA.
The redshirters I've met did it only for boys born within 3 months of the start deadline. I haven't met anyone who redshirted more than that. I do think it is a good idea for many boys, who frankly tend to be in the first few grades more immature emotionally and have poorer fine motor control. IMHO.