It's not a hard cutoff. Channel following (which adds important news from Partnered and Verified servers to your server) and the Friends tab should fulfill some of the key roles of the Activity Feed. And if you buy or otherwise receive games from Discord, the Library tab will remain unless you turn it off.

The company isn't shy about its reasoning. It doesn't want to include features that bog down the app and "take away from the core chat experience," and both the Activity Feed and Library were apparently guilty of that. Really, Discord is acknowledging that you use Discord for the community and in-game voice chat -- anything else is either a bonus or a hindrance.