Please test your plugins and themes against WordPress 5.0 and update the Tested up to version in the readme to 5.0. If you find compatibility problems, please be sure to post to the support forums so we can figure those out before the final release. An in-depth field guide to developer-focused changes is coming soon on the core development blog. In the meantime, you can review the developer notes for 5.0.
The WordPress open source project has evolved in progressive ways over time — supported by skilled, enthusiastic developers, designers, scientists, bloggers, and more. WordPress provides the opportunity for anyone to create and share, from handcrafted personal anecdotes to world-changing movements. People with a limited tech experience can use it “out of the box”, and more tech-savvy folks can customize it in remarkable ways.