Steel stair stringer size

It depends on how the particular stair was constructed. Although the absence of a central stringer is evidence that the treads are capable of spanning without the support of a riser, it is not proof that they are adequate for the span. Likewise, the absence of a central stringer does not mean that the stair is laterally stable without risers.

Even assuming the stair is constructed to withstand the removal of the risers, the feasibility of 'simply cutting the tops of the stringer' has a lot to do with what constitutes feasibility. Risers stiffen. Bouncing treads or squeaky steps are not consistent with the intent. What performed well before, may not perform well after modification.

In addition, aesthetics probably plays a role and might even be the driving motivation. Just having open risers is unlikely to be enough, they need to look good, and the ease with which the desired results can be obtained is probably also a consideration. Sawing and cutting and patching a closed riser stair to the point of an aesthetically satisfactory open riser stair is likely to be extremely labor intensive. Particularly given that errors are part and parcel of the learn as you go method one must use when tackling a problem without a straight-forward solution.

So sure, it might be feasible. That doesn't necessarily make it cheaper or easier or more cost effective than simply replacing the existing stair with something close to what you actually envision. Cutting out the risers doesn't change the basic fact that it's a construction project where quality of fit and finish matter.

Given that your stringers are going to be about 8" wide, you'll be fine with some pretty thin stuff. Heavy Jeep frames from the 1970s were only about 3/16" thick, and only about 4" wide. I've seen truck frames around 1/4" by 8", but they were larger single-axle agricultural trucks. Really large single-axle trucks only have about 1/2"-thick steel frames.

Come to think of it, I've walked stairs made with no heavier than 3/16" stringers in penitentiaries. They felt nearly like STONE.

If you're concerned about racking, remember that every tread will act as an anti-rack brace. Until your treads go on, the stringers will feel pretty flimsy if you push them sideways. AFTER the treads go on, they'll be train-wreck rigid.