I found this by searching for "indices":

https://docs.python.org/3/reference/datamodel.html?highlight=indices#slice.indices

Slice objects support one method:

slice.indices(self, length)

This method takes a single integer argument length and computes information about the slice that the slice object would describe if applied to a sequence of length items. It returns a tuple of three integers; respectively these are the start and stop indices and the step or stride length of the slice. Missing or out-of-bounds indices are handled in a manner consistent with regular slices.

The documentation for the slice class should mention the indices method. It is a documentation bug to say that slice objects "have no other explicit functionality".

The comment "they are used by NumPy and other third-party packages" seems to be irrelevant. Most builtins are used by numpy and other third-party packages. I think that comment might be a left-over from ancient Python 1.5 when extended slicing with a step had been added to the language for numpy but no builtins used it yet? Maybe I am misremembering.

Either way, I think we can also remove the bit about numpy and third party packages.

You're looking at the built-in functions page. That's the wrong place. That doesn't contain documentations of the whole types. The built-in types page does. The slice type doesn't have its own section there, but it's listed in Internal Objects, which refers to the data model page, where the slice type and this method are indeed documented (as shown by @hugovk above). Looks fine to me.

On the built-in functions page, set() starts with "Return a new set object", linking to the built-in types page. While slice() starts with "Return a slice object", linking to the glossary. Maybe that link should instead equivalently go to the data model page section about the slice type.

Thanks, I've updated PR #99129.

I also removed the NumPy bit, please let me know if that should stay, and if the text should be reflowed.