I'd like to hear your definition of a "soft cap".
The old system was:
1st leg: base chance + cumulated chance from bad luck protection
2nd leg: base chance + cumulated chance from bad luck protection
3rd leg: base chance + cumulated chance from bad luck protection
4th leg: base chance + cumulated chance from bad luck protection
5th forwards: base chance
If there weren't ANY cap, the chance for EACH leg would be the same. Is it the same? No, after 4 the chance is different.
Is there a hard cap on getting legs? No, you can get more than 4. Unlikely, but possible.
Therefore, there is a soft cap. Your chance on getting the first 4 while playing is higher than getting the next 4.
To put it into one sentence:
A hard cap on the bad luck protection resulted in a soft cap on the amount of legendaries that might drop within a timeframe.