I think that’s a heavy duty banister bracket, like this one:
Some banister brackets have a swivel type arrangement so that the bracket can be vertical underneath the handrail, no matter the angle of the handrail. Basic heavy duty brackets like this one are completely fixed in orientation, so they’re installed with the vertical support at an angle to support the handrail. I suspect the bracket in the photo is at a 45ish degree angle and only looks vertical due to the perspective. The banister has been deliberately cropped out of the photo to make the perspective as confusing as possible.
Personally I think the photo was taken from the top of the stairs looking down, based on the wear in the carpet.
People who are too cheap to refinish old stairs. Source: I bought a house that had carpet on the stairs, and when we pulled up the carpet we discovered a worn out eyesore that cost thousands to update. In our case the previous owners were a cash strapped family of two young kids with a third on the way. They needed a bigger place for their family so I don’t fault them for going cheap on the stairs. The all-white paintjob and industrial grey carpet in the photo scream “landlord special” to me, so I’m less inclined to be forgiving in my assessment of the stairs in the photo.