It looks as though the top of the stem was badly damaged and has regrown a new shoot (the healthy part above the scar). Robinia are very brittle and prone to snapping. You could prune it off to below the scar and let it regrow. Or put up with it, it may eventually heal over but could be a place where infection could get in to the plant.

It looks to me as though the stake is perhaps too long, and during windy weather it has rubbed against it and gradually, through winter, worn away the bark.
As Volunteer says infection could get, but alternatively it may just get through ok. I would definitely shorten the stake, it only needs to be long to stop the roots moving.

I wouldn't shorten the stake now unless you're going to chop the tree down past the point of this fracture - that area, though not looking infected or unhealthy currently, is a weak spot and will be prone to break and crack more if its able to move around too much. I'd keep the stake the length it is, as I say if you are not going to prune the tree back - it's acting like a splint.

It looks like the main stem broke, and tore a long strip of bark off of the trunk, possibly as much as a year ago, it's healed that much. I'd remove the stake next to the trunk, and put two stakes on opposite sides, about 1/2 meter from the trunk. Tie it somewhat loosely to the stakes, so it can sway in the breeze, but won't bend too far in serious wind or snow load. The swaying motion helps the trunk to thicken up, and also helps the wound heal faster.