And what exactly would an "interpolated slide" be?

I mean using interpolation to make volume slides sound smoother when transitioning between the individual volume values, or to make panning slides smoothly transition in-between the panning values. Like instead of jumping between a bunch of little points, it is a gradual ride between them.

You can already make any slides much smoother by using modern tempo mode and choosing a high tick count, e.g. 16 ticks per row.

I tried what you suggested with some existing modules, and all that really does is make the slides faster (not necessarily smoother), and thus if set too high it will make "fade-outs" and "fade-ins" more abrupt.

If you want to retrofit an existing module, you have to edit it of course and replace the slide parameters by more appropriate ones.