Ditto that looks like an old injury.
One of my horses had a scar like that when I bought him six years ago and he still has it. He's been barefoot five years. It's never caused him problems but his trims never go past four weeks and I can't ride anymore elst I would either boot him or put shoes back on him.
It has always been closed and never caused him problems but the Seller told me their dominant Bee-atch of a mare ran him thru the fence when they first brought him home.
And yes, since the old owner thought "horses hooves will take care of themelves", I am also glad you have him. Good luck in the rehab