For those that are not familiar with them, recently Delicious added a new feature where you can bundle a bunch of links together in a Stack. This, I suppose, has a similar use-case to Tag Bundles but rather than grouping bookmarks automatically using tags, you manually add them to a Stack.
It seems that they are a popular new feature.
Unfortunately, and the reason that they’re not in Yummy, is that they have not yet provided developer support for Stacks. There is no way that Yummy, or indeed any third party client, can request the information from Delicious.
If you would like to see Stacks in Yummy, I would suggest that you ask Delicious to allow API access to Stacks. While I can’t say how long it would take to implement and release, I can say that it’s a feature that I would like to see in Yummy myself.