[darekkay]

Check their status page [1] to see the progress:

> Investigating - We are currently investigating 403 / 429 errors received by some users.

[1] https://status.npmjs.org/

[ORioN63]

It seems the issue is resolved at the moment. From the same link

> Monitoring - Our content delivery partner has informed us they've implemented a fix. We are monitoring. Feb 17, 13:07 UTC

Edit: And in the OP GH thread, people are also mentioning it's back up.