extensions have become gems, and the engine itself has become a collection of several gems: spree_core, spree_auth, spree_api, spree_dashboard, spree_promotions, spree_sample
site_extension is no longer needed to set up a specific store, now all settings can be made directly in Rails.root
“Vendor mode” is excluded from the deployment methods, since thanks to the Bundler, there is no longer any need to clone the Spree sources into the project directory, even if you use the edge version or your own fork
In addition, the work with payments has been improved, the work with surcharges has been simplified, and of course many bug fixes and minor improvements have been made. ')
Official Release Notes