Why they shut it down? I do not know. But I have several web applications running myself, and maintenance costs will add to the total budget. Since they have a new license system now, I guess they wanted to do some housekeeping.
C4D versions came in my mailbox, up to, and including R17.
After that, they had to be downloaded. If you are a registered customer, I see no reason why you cannot get your SN and setup files.
I got links in my support response, from Maxon, links to everything, setups and SNs for my R18, R19 and R20.
Here in Norway, C4D was sold through a local dealer. I assume that is the case for you too, isn't it? Contact the dealer, the least they could do, is to help you out here, if Maxon is slow.
-Ingvar