You might need a small mirror to see it, but is there a similar notch on the underside, opposite where this one is? It does look like a deliberate cutout to clear something on another application for the same part, and not something that has been "broken"off.
That is based on the radius and relief angle all around the rest of the flange that results from the punch to die clearance of the blanking process during manufacture of the part. But the rusting obscures this so I can't be positive. Someone else will chime in if they have the same thing on theirs, I'm sure.