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.