FWIW, it's a grey area that has no hard and fast rules to govern the practice. All we have are recommendations from manufacturers that want to cover their butt, and examples with, and without, damage. There are many vintage guns that have been used (not by me) for steel for years with no apparent damage, but each one is it's own case study. We accept that chokes tighter than modified increase our chances of barrel/choke damage, but Worth Matthewson, the noted waterfowl writer from the Pacific Northwest used an L C Smith for years as his only duck gun, after steel shot was mandated. It was choked very tight full, in both barrels. He took it to a gunsmith for some other little problem and was advised that the chokes were very tight and that he should be careful with steel shot in them, but there had been zero damage. He wrote about it in his book Big December Canvasbacks.