It's relatively feasible today with (UI|NS)BezierPath and CAShapeLayer. It's not perfect in every situation, as CAShapeLayer is optimised for performance, but in the vast majority of cases it's worked well for me. Especially when paired with tools like PaintCode.

The tools are there by Apple, but I don't think the hardware until fairly recently could really justify the performance hit - instead choosing file size over performance. These days though, in the majority of cases, the performance hit is negligible (especially when compared to dev/design time of generating @3x assets years/months later)