Hi, an index exists here at the .deb files! However, access is denied here at the archives (.tar.gz)!
It would be better if the last index had the same structure and was open like the first one
These links might be what you are looking for, it will list downloads for package managers and tarballs.
Current release for each major version:
https://downloads.mongodb.org/current.json
All releases:
https://downloads.mongodb.org/full.json
The index would be better