I have the same issue with the Subscribe button that Thomas has. It seems that I am only able to subscribe to all downloads for all products and all versions from the top-level page.
It would be helpful if it was easier to customize your subscription more. For example, I would like to subscribe to all security-related downloads for all ArcGIS Enterprise core software (Web Adaptor, Portal, Server, Data Store, no extensions) across versions 10.4 - 10.8. It doesn't look like this is possible to do, other than to create 20 separate subscriptions. And, I'd have to create 4 more subscriptions for each new release of ArcGIS Enterprise.