About 5 minutes ago Chris Lambertus (Apache Infra team) who reported the issue to Sonatype forwarded this message:

Thanks for bringing this to our attention. We performed a round of DNS record cleanup today and unfortunately this record was mistakenly removed. We've rolled back the changes, though it may take a few minutes for the changes to propagate. I'll resolve this issue once I have confirmation the name resolution is working again.

So as soon as DNS propagates to your systems it should be available again