Picked up a 28" bbl with modified choke to repurpose my Deerslayer. Safe to use steel? Probably #4 and smaller.

If it's an old one I wouldn't. However, you can call Ithaca in Ohio and get the right answer.

I'd find an extra barrel and either have choke tubes installed or open the choke to IC and use it exclusively for steel. I've never had problems with using steel in fixed-choke shotguns that had open chokes. On the other hand, I've never used shot larger than steel 4s; I understand that the larger the choke, the greater the chance of peening or splitting the choke area. In my guns steel shot shoots a tighter pattern than lead, size-for-size, and IC is enough choke for most upland shots. Notice I said "tighter" not "tighter and more even."

I'd find an extra barrel and either have choke tubes installed or open the choke to IC and use it exclusively for steel. I've never had problems with using steel in fixed-choke shotguns that had open chokes. On the other hand, I've never used shot larger than steel 4s; I understand that the larger the choke, the greater the chance of peening or splitting the choke area. In my guns steel shot shoots a tighter pattern than lead, size-for-size, and IC is enough choke for most upland shots. Notice I said "tighter" not "tighter and more even."

I did the choke tube/steel suggestion. After a couple boxes, the tubes must have expanded and the threads got crushed. It's still in there and has little indication of wanting to come out.

Then again, Ithaca may make steel compatable choke tubes. Simple as a call to the source.

Took less than 5 minutes with Ithaca.

Full is the only fixed they don't recommend for steel. Have at it with Mod.

Yes they make steel compatible chokes, but don't recommend using full.

If it says Roto-Forged, then go for it with steel. I know we have for a couple decades with no problems.

An important point being left out is gauge... small gauges and steel are a bad idea... the tighter the choke and gauge and the bigger the shot the harder steel is on the barrel...

Installing tubes designed for steel in older guns works very well, but you still should shoot the smallest shot you figure you can get away with.

Big shot, tight chokes, and small gauges is absolutely the wrong way to go.

My bad in not providing more details. Its a 12 ga rotoforged barrel. Thinking I am going to run the steel if required where I am hunting.

Modified 12 gauge with 4s will probably be fine.

We've shot plenty of T shot through that barrel for years with no problems.

I have no idea where "plenty" starts for you, but I would not shoot T in anything Mod or tighter... and probably not Mod...

It's already been being done, since 1986. No problems yet.

I don't shoot T much anymore, but we shot a lot back in the day. It still runs BBs every year.