What is up with Texas redshirting?

I have a friend whose child turns 6 in April and is still in preschool, meaning he’ll start kindergarten at 6 and a half! That’s fully first grade age. She said it’s super normal in Texas to redshirt spring birthdays! Huh? I mean, this is getting ridiculous right? I get they do it for sports over there but wow. My kids are in K and don’t even turn 6 until summer vacation. I couldn’t imagine if kids were turning 7 in their K class!