I would be shocked if GitHub rate limited this new package registry. They're just serving tarballs and static content, and it's a new system so they can fully architect it with scale in mind (i.e. a CDN). They rate limit current repository-related content because they have to dynamically generate most of it in response to requests (I assume they have caching here as well, but not static-file-behind-CDN level caching).
I think, as many have said, that this is going after GitLab more than anyone else, although I can see a lot of users migrating away from Docker Hub given 1) the latest snafu/breach and 2) why keep my container repo over here and my container build pipeline over there? Doesn't make any sense and Docker Hub doesn't come with the pedigree of CDN baked in. I'm sure the same arguments work for other technologies in this consideration, but... Docker seems to continually be behind the 8-ball on the shifting field. My guess is Microsoft buys them in the next 3 years at a discount anyway. It fits their pattern of getting in front of the modern ecosystem and since Docker has leverage with containerd right now it would be an unsurprising move.
In general, I find that caching/vendoring dependencies is the most sane thing to do, but it's not what, say, the Javascript world appears to be doing. Do we want to move towards a service that already rate-limits package fetches when we already have a service that doesn't?