That steerer tube getting notched like that is really unusual. I can understand the concern whether it weakened the fork enough to become unsafe to use, as in most cases, introducing a "step" on metal parts can result in stress risers where cracks can start. Question is, what is the type of stresses that goes through that area when riding.
Along with compressive forces/shock going through the tube, I assume the are beding/moment forces also going through where the top of the fork crown meets the steerer tube when that notch is located. IIRC though, the steerer tube on most bikes are butted where the fork crown meets it, so they have tucker walls in that area.....maybe enough to compensate for the weakening that the notch might have caused??,...... I dunno.....