We don’t design for what’s normal. We design for the unusual. Just because you don’t have wind storms too often, doesn’t mean that you shouldn’t take precautions for high winds.
Structural shear walls are usually at the exterior (perimeter) of the house. However, perimeter shear walls can only resist a certain amount. If the house is more than 3:1 length to width or if it has an unusual shape (like “L” shaped) or a split level house, then interior shear walls are often required.
You call the diagonal brace a “beam”. I call it a diagonal shear brace. (The code allows diagonal shear braces, in lieu of plywood shear walls.)
I suspect the diagonal shear brace is structural, because someone went to a lot of work to carefully cut the studs for the diagonal brace. One way of checking to verify that it is structural is to 1) verify that it extends from the top plate to the sole plate, 2) verify that a diagonal brace or shear wall (plywood or OSB board) extends up to the roof in the attic DIRECTLY above the diagonal brace, 3) verify that a diagonal brace or shear wall extends down to a CONTINUOUS footing. (Are those 2-16 d nails into each stud?)
If any of these conditions do not apply, then it’s not a structural shear wall FOR WIND OR EARTHQUAKES. (However, it could be for lateral stability for stairs or some other element. To verify that, we’d need more detail.)
