Krita is a raster drawing program in which drawings (used in animations) are rendered on the canvas by reproducing the stored color values in the underlying individual pixels.
Inorder to implement interpolation, a sample of outline points (which are used to encapsulate an animatable object ) need to be identified by the underlying software and then math operations need to performed , based on the start and end location of the individual points of this object's outine .
This is achieved in vector drawing programs like Animate and ToonBoom by employing mathematical formula's which define the outline of the object and then the change in the position or shape is interpolated by determining the change in these outline values and then interpolating the inbetween values.
However,as Krita is a raster application ,the outline values are just color values stored in individual pixels and the most one can do to these values is to change it's color or opacity and therefore no shape or position interpolation is available in Krita. Krita only provides the ability to interpolate the opacity of the layer as seen here
However,now that Krita is also providing Vector graphics support , there may be a chance that some time in the future ,shape and position interpolation may become a reality.