For steel frames the fatigue life is pretty much infinite provided there aren't stress raisers in the frame. This is fundamentally different to Alu, although if well engineered (Zaskar ) it's as close to infinite as makes no difference.
And no, the frame isn't getting softer, it's just that Alu bikes are built stiff to give them acceptable fatigue life, so we are all used to hyper-stiff frames these days.
Whether a very stiff frame is a good thing is a separate discussion. Bike brands promoted this as an advantage, whereas it was more of a consequence of the move to Alu (which is cheaper to fabricate than steel) than a specific design objective.
And no, the frame isn't getting softer, it's just that Alu bikes are built stiff to give them acceptable fatigue life, so we are all used to hyper-stiff frames these days.
Whether a very stiff frame is a good thing is a separate discussion. Bike brands promoted this as an advantage, whereas it was more of a consequence of the move to Alu (which is cheaper to fabricate than steel) than a specific design objective.