What is it called when there is natural truncation selection?
Truncation selection

The term  truncation selection has been used a lot in theory papers because it is a simple model and because it is of interest to breeders. Truncation selection is most likely more common in a breeding setting than in nature and from wikipedia,  truncation selection is defined as specific to breeding


  In animal and plant breeding, truncation selection is a standard method in selective breeding in selecting animals to be bred for the next generation


However, as much as the definition fail to repeat animal or plant instead of just animal, IMO it fails to recognize that such notion could as well exist in nature. I would personally not be shock to read a paper talking about  truncation selection in wild populations.

Threshold model

If you really want to avoid talking about truncation selection, then you can eventually picture the phenotypic trait as a liability factor (as it is done in sex determination literature, see Perrin and Beukeboom's book) and selection acting as a threshold on this factor. Note that the concept of threshold and threshold model is very common and everyone will understand what you mean.