Looks like it is removed to force developers to use equal_slice_bits function. This function name is better in that it highlights the fact that only bits are compared and not references.

This is the old definition of both:

;;; Checks whether the data parts of two slices coinside
int equal_slice_bits(slice a, slice b) asm "SDEQ";
int equal_slices(slice a, slice b) asm "SDEQ";

So these were both equal functions.