Have you consulted a structural engineer or a staircase pro?

What are we talking about here cutting back about 10" give or take? What will you gain by doing that... What are you planning to do with that space?

Enclose it put a door and you will have some storage space away from everything.

An engineer is completely unnecessary. That's like asking a brain surgeon to trim a fingernail.

While the framing is structural, I'm not seeing any harm in removing the circled area, as the header supporting the landing remains supported. Maybe a couple straps/hangers to prevent twisting, but the house is hardly going to fall down.

Yes- gaining about 10” by cutting 10”. There will be a wetbar on the other side. I’m going tuck a refrigerator under the stairs and that wall sticking out forces me to move it up further.

I am still confused as to what you gain I think what Joseph stated is probably true but I would need to see a to scale floor plan to undestand how removing 10" makes a difference .I see a rough in for a toilet I think so maybe a 1/2 bath would be a better use of that space .

The OP wants to cut it out, so who are we to question what he will gain. A scaled floor plan AND an engineer are totally unnecessary. The stair landing is already adequately supported. Cut it out with a reciprocating saw.

@RappArchitecture "so who are we to question what he will gain"

Seeing the exposed portion of the stairs coming down, the landing, etc and the OP didn't mention what he was trying to do with that space... I was trying to understand the purpose of removing 10" or so of that wall and leaving all that exposed up there, which still needs to be covered to avoid seeing all that... So I thought maybe there is a better solution or a work-around to avoid all that work.

You can truncate the wall back flush with the post under the corner of the landing as said. However, if you are that in tune to gaining 10" there (from the pic I estimate 5 stud widths beyond the post, so 7.5"), I would think you would be just as concerned about all the soffit work and angled stair run above. It's a very messy visual and will probably be more noticeable with a solid row of cabinets somewhere there.