While they haven't announced a release date yet, they do have it listed on the Product Roadmap as an upcoming release in Q4-Q1 of 2024. The multiple due dates are being referred to as "checkpoints" by Instructure. I think this is to avoid confusion with assigning multiple due dates with the Assign To feature. The item on the Product Roadmap referring to this feature is the following: "Students engage in deeper, more meaningful discussion with classmates with clear checkpoints to guide their way."
In the comments of a recent blog post about the Discussion Re-design, you can see confirmation from Instructure employees that this feature is planned for development. @SamGarza1 said they will be sharing more updates about this feature and others in the future, so it would be good to keep an eye on Release Notes/Deploy Notes and the Product Blog. You can also follow the prioritized theme for improving discussions to try to keep up to date on any new information shared there too.
