Yes, steel has a "fatigue limit," but this irrelevant when it comes to bicycle frame life. There are two major reasons for this. First, bicycles, aircraft, etc, built with aluminum are designed such that normal fatigue should not cause a failure in a lifetime of use. That doesn't mean fatigue failure is impossible, as it depends upon the frame, how it is assembled, damage and a host of other factors. Second, although steel itself has a fatigue limit, steel frames almost certainly do not and have a finite lifespan just as aluminum frames do. Steel frames absolutely accumulate fatigue stress and do eventually fail from it. Yes, including notoriously overbuilt Surlys - I've heard of multiple Surly frames that eventually failed from fatigue. That's not because there's anything wrong with Surly frames, and they are certainly not dangerous. But a frame of joined metal tubing is absolutely subject to fatigue stress, no matter what it is made from.