The official development discussion platform is devtalk.
This forum is used for developers to discuss technical topics, get user feedback on features under development, and for new developers to get involved.
Contributions to translation and documentation can also be discussed. It's all about collaborating to improve Blender.
Given the much larger number of users than developers and to avoid drowning out that discussion, features requests and help using Blender is off topic. For these see Feature Requests and User Communities instead.
Active developers are encouraged to subscribe to the Announcements category on devtalk for important information.
Until 2023, mailing lists were used to coordinate development, and were progressively replaced with the DevTalk forum. The lists archives are available at archive.blender.org.