There are many non-official themes for window decorations in KDE store, which change how the window buttons look like, their spacing, and might change the title bar color, but other aspects such as the shadows or the softened borders are only defined for the official KDE themes (breeze and oxygen). As such, if I go and choose a window decoration that is more of my liking, I can no longer get a shadow or other effects. I think having the smooth borders independent of themes might be a bit too hard, but the shadows that appear around windows should be something that could be applied to any theme. Would be nice if one could configure the same shadow effects from breeze in custom window decorations.
They cannot really be independent, because the shadows have to follow the way the decoration is rendered (e.g. regarding roundness). But maybe kdecorations2 library could offer a shadow engine that decorations could optionally use, instead of forcing them to implement their own. That probably won't help for the Aurorae themes, i.e. most of the available decorations.
I doubt there is any value behind splitting decorations and shadows. Decorations and shadows are interconnected. If one wants to have a different shadow, then they need to fork the decoration theme and change the shadow accordingly.