Slice Luminosity does indeed remove an image’s alpha channel, this by design.
For background information, see:
- Manually set luminance channel values?
- Is it possible to select a tonal area in histogramm in a filter?
Typically, this filter is used on a single image that ultimately comes from a camera, so an alpha channel isn’t expected, and is removed if found; in default behavior it uses the alpha channel during the creation of a mask image.
In operation, the filter lets you specify (at least one, and as many as four) luminosity ranges that might interest you. In its default input/output behavior, the filter alpha-masks out all but the regions of the image that fall within the specified luminosity range(s).
This masked image is created as a new layer; the original image is retained as the background layer. The typical workflow is to modify the masked image, then flatten layers into an aggregate image.
Because it can use the alpha channel for masking purposes, it removes previously created alpha channels.
Hope this helps.