Back when I first lost access to the JSU, I had a recent backup of all my music, but I hadn't kept a copy of the JSU.
At that time, the iTunes support website already stated very clearly that they had a policy of not allowing purchased content files to be downloaded a second time, and that backups or re-purchasing were the only two options.
This was a very bad thing with regards to the JSU because, at the time, it was only possible to ask to purchase the JSU once. If you attempted to pay for the JSU a second time, you couldn't get past an error message saying that you'd already bought it, therefore there was no need to buy it again. (There is no such error message when you attempt to re-purchase music.)
It's an even worse thing right now due to the fact that, as noted, the JSU doesn't appear to be available for purchase at all anymore.
Notwisthstanding that published policy, they were still accommodating to me at the time, and allowed me to download the JSU a second time.