I may have misunderstood a lot. Here's a set of BTZS curves for Ilford HP5+ developed in D-23 1:1 at 70 degrees. They come from a BTZS film test done for me by Fred Newman of the View Camera store, and they are consistent with my own test results. He exposed the step-wedges, I developed the film, and he scanned and analyzed the data.
From left to right, we see contrast curves for 5 development times: 16 min, 11 min, 8 min, 5.5 min, and 4 min. (This chart shows more information too, like the Effective Film Speed at different development times. For example ISO 250 when developed for 8 minutes. It's a predictably ho-hum result, showing that HP5 and D-23 1:1 is a fairly mild combination - which I like because when developing by inspection, longer developing time affords greater control when handling many sheets at once.)
The only curve that shows a trend towards shouldering, is the 4 minute developing time, which represents an N-3.5 development. It's so short, I'm surprised we see much of anything.
Its my confusion. Anyway, here is the Ilford curve from the data sheet showing the curve.
I think all charts would show this leveling out if they extended so far just at what density I'm not sure.