You bring up a great point.
I actually spoke with 2 different representatives with Remington before I bought the gun. Both assured me that any 870 barrel that had ever been manufactured was compatible with modern steel loads. I gave them the year, serial, everything. I thanked the first guy and assumed he was full of you-know-what. I called the next day and got a different rep. He said exactly the same thing. I asked point blank: "So Remington's official stance is that it is safe to shoot steel out of a 1953 wingmaster?" He never skipped a beat, and stated that they had never been able to replicate barrel failure, even though they had heard stories. He stated that even though it was an old gun, it was relatively modern in design. He said a new 870 barrel was basically the same beast as on old one. He did say they didnt recommend anything tighter than a modified choke, and that the old chokes were not universally approved for steel.
I know my buddy has an old A5 that he shoots bismuth out of. I'd like to try it, but I've shot over 100 rounds of high-brass steel with no evidence of anything being awry. It's an IC, so theres that as well.
I'm not saying you're wrong. It may banana on me this weekend. But I did check with the manufacturer.