Rack::Session was moved to a separate gem. Previously, Rack::Session was part of the rack gem. Not every application needs it, and it increases the security surface area of the rack, so it was decided to extract it into its own gem rack-session which can be updated independently.

Many of the types here should have been built-in. You can help by suggesting some of them to the TypeScript project.

They reduce the distinction between a “core module” and a “plugin”

Each library is maintained in its own repository, allowing decentralized ownership and independent release cycles.