It's raised by @a-sully in CL review, thanks!

Softplus calculates ln(1 + exp(steepness * x)) / steepness, when the steepness is 0 it might result in division by zero.

I tried Pytorch torch.nn.Softplus(beta=0) and the results are all inf, and TF and ONNX don't have this attribute. Also DirectML doesn't support steepness < 1.0.