Daniel Vrátil wrote a blog post today about Akonadi "still alive and rocking". The first Akonadi release based upon KDE Frameworks 5 happened a few months ago while there is no assurance of API stability yet. Since the KDE4 days, Akonadi has switched from a text-based protocol to a binary protocol for better performance, various new features were added, and other progress made.
For more details as to the state of Akonadi and looking ahead, see Daniel Vrátil's blog. If you missed it today, also be sure to read Plasma 5.5: The Quintessential 2016 Review for all KDE fans.