Relatedly, some iCloud storage plans were unavailable for purchase on iOS and macOS Sierra. When we checked the "Upgrade iCloud Storage" menu on an iPhone, both the 50GB and 200GB tiers were no longer listed. Later, the 1TB tier disappeared as well. On another iPhone, the menu wouldn't load at all.
Apple has not announced any changes to its iCloud storage plans, and it has not recently updated its iCloud storage plans and pricing support document, so this was likely simply a server-related issue on Apple's backend. Apple's System Status page does not reflect any ongoing iCloud-related issues.
Apple has not indicated why the "discontinued" emails were sent, but all four of the 50GB, 200GB, 1TB, and 2TB tiers appear to be available for purchase again for most users. We'll update this article if we hear anything else, particularly if Apple does have surprise changes to its iCloud storage plans in the works.