Some repositories have '+' on the url.
After a search like this one libstdc++: https://archive.softwareheritage.org/browse/search/?q=libstdc%2B%2B&with_visit=true&with_content=true
The proposed links send the user on page returning a 404 error : https://archive.softwareheritage.org/browse/origin/directory/?origin_url=http://snapshot.debian.org/package/libstdc%20%20-arm-none-eabi/
If the %20 are replaced with %2B, the repo is found:
https://archive.softwareheritage.org/browse/origin/directory/?origin_url=http://snapshot.debian.org/package/libstdc%2B%2B-arm-none-eabi/