They were accused of that by people who didn't understand that batteries degrade over time, and the resulting legal suits were entirely about disclosing the throttling, not the throttling itself. Newer iPhone models still do the exact same thing, they just provide more information about it, and let you toggle it off.

The idea that they were doing this maliciously never made sense anyway, customers who haven't upgraded in a while might be the least lucrative audience to target.