Reviewed all APIs to make sure we do not expose implementation classes as part of our public versioned API. We will make a separate post about this with more details.
Some of the smaller breaking changes:
Links to unpublished content will return 404 (instead of login screen).
Localized texts will fallback to a fallback culture in the language files before falling back to values specified in code.
LanguageBranch class now implements IReadOnly to support read-only caching.
No longer possible to create sites without at least one host defined.
Properties PageData.StartPublish/StopPublish are changed to Nullable.
StartPublish will be null until published or set explicitly (today it defaults to the created date which is misleading).
Fixed confusing handling of empty properties (for example assigning an empty link item collection to a typed model resulted in a null value)
IContentVersionRepository List methods have been consolidated into one
The UI Framework EPiServer.Shell.dll will be moved from NuGet package EPiServer.Framework to EPiServer.CMS.UI.Core.
A lot more details will be provided as part of the release, including a list of all breaking changes and which APIs are being changed. Pre-release packages of the breaking change release will be made available as part of the continous release process in the coming weeks.