A "linear plateau" usually means that the explanatory variable is measured on a continuous scale ... or at least at more than four points. When the X variable is measured at many locations, you can use a "segmented regression" to estimate the point at which the data flattens out. An example is shown in this article on segmented regression in SAS.
Personally, I don't think these data indicate a plateau. Graphing the data does not indicate a plateau:
proc sgplot data=Have;
vbox y / category=x connect=mean;
run;
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