When you uploaded your code to GitHub you granted them a license to host it. You can’t use DMCA against someone who’s operating within the parameters of the license you granted them.
GitHub's terms of service specify the license is granted as necessary to provide the service. Since the service is not provided they don't have a license.
Hosting the code is providing the service, whether you have a working account or not.
Also was this code open source? Your stack exchange contributions were open source, so they don't need any ToS-based permission in the first place. They have access under CC BY-SA.
Some, not all. GitHub is unlikely to continue hosting the code on the basis that it's open source. If they do, I'll send them a GDPR request to detach my name from it, including in source code comments and package names.
It's not always clear that Stack Exchange always followed the CC license, and if they violated it once, it was terminated. The checkbox you have to click now to access the data dumps might be a violation. The data dumps don't come with copies of the licenses, so that's a violation.