Keeping the source available for projects, even those don't migrate to GitHub or elsewhere, is important to us.
We've updated the blog post with some additional details on the long-term archive plans: After December 15th, 2017, "CodePlex.com will start serving a read-only lightweight archive that will allow you to browse through all published projects – their source code, downloads, documentation, license, and issues – as they looked when CodePlex went read-only. You’ll also be able to download an archive file with your project contents, all in common, transferrable formats like Markdown and JSON. Where possible, we’ll put in place redirects so that existing URLs work, or at least redirect you to the project’s new homepage on the archive. And, the archive will respect your “I’ve moved” setting, if you used it, to direct users to the current home of your project.
There isn’t currently any plan to have an end date for the archive."