I had a mare that got the exact same injury as a youngster, but hers looked like it was a bit deeper/wider. She had no problems with it when she was being used as a broodmare, it grew out with a line, but never a crack. However when I started using legging her up again it began to grow out as yours did with just a small crack. When I began hard daily riding, it suddenly cracked open much wider during a ride and caused her to be lame. I tried growing it out naturally with supplements/boots but ended up having to put a shoe on to support the hoof. After just a few months of very strict trimming, and shoeing, it grew back without a crack. I didn’t ever have to put shoes on again because I kept her trimmed short at regular intervals. A boot wouldn’t help because it’s not holding the hoof together to keep it from being stressed.