As an example, take this question on Programmers. I suggested a possible related question and eventually 5 users closed it as an exact duplicate. I'm not sure if it is a duplicate, but that's not really the problem. I also found two other questions that, when all three are combined, probably answer the questions of the asker. I posted these links in a comment, which also makes them show up in the side bar as a "Linked" question. However, this isn't really that visible. Previously, I would have added all of these links to the box that listed the duplicate that it was closed as - this made all of the helpful questions stand out to not only the asker, but any future visitors.
I'm not sure if this should be a high-rep or a diamond-mod function, but the ability to add links to other questions to the "this question already has an answer here" box should exist for at least some users. Perhaps even limit it to 3-5 questions, so it doesn't become a list of kinda-sorta related questions.
I'm calling this a bug because we lost functionality that we used to have. Perhaps it should be a feature request, given the change in duplicate functionality?