Is it normal for scaffolding on a job site to be this rusted through?

You're not overreacting. Rust holes you can nearly stick a finger through at a joint means that frame's wall thickness is compromised and it should've been pulled from service a long time ago. Scaffold frames are supposed to be inspected by a competent person before every shift, and damaged ones get tagged out. If nobody's tagging anything on that job, that tells you plenty about how safety is run there. I ran into something similar a few years back on a restoration job, except it was bent base plates and cracked welds instead of rust. Told my foreman, got brushed off same as you. What actually worked was walking up to the GC's super during lunch and showing him. Took him about thirty seconds to look at it and he had the whole section off the site by end of day. GCs carry the liability on that job. A collapse that drops three masons with mortar boards full of brick is a lawsuit and a work stoppage they want no part of. Most supers will act fast and they won't burn you to the sub, because guys who flag real hazards are useful to them.

If the GC super blows you off too, then yes, OSHA. You can file a complaint online or by phone and ask for an inspection, and you can do it without your name attached. There's also a rule that protects you. You have the right to refuse work you reasonably believe is dangerous, as long as you asked them to fix it first and there's no time for an inspector to get there. Document everything, photos of the rusted joint, who you talked to, when. And honestly, don't climb it in the meantime. No brick job is worth a five story fall. If they push back on that, you've learned exactly who you're working for.

Agree with all that, one thing I'd add. Take pics with your phone and timestamp 'em before you do anything else. I got brushed off on a job years back and the foreman claimed later I never said a word about it. Photos of what you flagged and when shut that down fast if there's ever an injury or a workers comp fight. Also worth checking if those frames are rented. A rental yard does not want rusted junk out on a jobsite and a call to them sometimes moves things quicker than going through your own chain.

One thing to keep in mind is the load side of this, because it's worse than it looks on paper. A frame that might technically survive empty is a diferent animal when every guy climbing it is hauling up a hod of brick and a bag of mortar, and those loads cycle every single morning. Corroded steel fails in a wierd way too, the outside can look intact while the wall thickness underneath the flaking scale is gone, so eyeballing it from the ground tells you almost nothing.

If you do end up calling OSHA, the scaffold rules live in thier construction standards under the masonry and scaffolding sections, and the inspector will look at exactly what you described. I'd also ask the super in writing who the competent person on site is, because the answer to that question usually ends the arguement one way or the other.

One trick that worked for me years ago: I grabbed a flake of rust off the joint, bent it apart with my hands, and dropped it on the super's desk. somethig about physically holding the failed metal lands different than pointing at it. That crew was four frames up the next morning on new legs, and the old ones went back to the yard. Also, don't assume the job's 'competent person' is actually qualified. I've seen that title handed to whoever's been there longest, not anyone who knows what pitted steel looks like up close. If the tag on that scaffold is years faded, that's your answer.