OpenNMS Horizon 23.0.0 is the first release of the stable 23 series.
It contains a number of bug fixes and enhancements, including support for alarm correlation, running OpenNMS components in an external container, and many other improvements.
For a high-level overview of what's changed in OpenNMS 23, see What's New in OpenNMS 23.
The codename for 23.0.0 is Granny Weatherwax.