The canonical location was: https://bitbucket.org/quaelgeist/roundup-drag-and-drop
The last change was in 2016 and it is no longer there as of Oct 2020. I attached a zip file of the repo here for safe keeping. The tracker is documented in the email: https://sourceforge.net/p/roundup/mailman/message/34914714/
It starts:
From: Sean Engelhardt <sean.engelhardt@in...> - 2016-03-07 20:51:19 Hello Roundup-users, I want to suggest you a little tool that I wrote about one year ago, I named it "Roundup Drag and Drop" (roundnd). This allows you to "sort" tables in roundup using drag and drop. Tables are sorted by a "sequence number" which have to exist in the tracker. After sorting, the sequence number will update itself according to the surrounding sequence numbers. Drag and drop-support is implemented using jQuery and jQueryUI. When sorting things, you might facing unexpected results. The idea was to be able to sort some kind of Scrum-backlog in roundup using this plugin.