This release appears to be very Logic-oriented, which is probably why we’re not seeing a MainStage nor GarageBand update at the moment. Below are the highlights. See the link for the complete list. It’s huge!

https://support.apple.com/en-us/HT203718

What’s New in Version 10.2.3

This update contains numerous fixes and enhancements including:
• Crossfades between comp sections in Take Folders can now be graphically edited
• Improves sound quality for Flex Pitch editing
• 7 additional plug-ins have been redesigned to add Retina support and improve usability
• New Loudness Meter plug-in provides support for LUFS metering
• Nudge controls can now be used to edit the position of selected automation points
• Icons for Drum Machine Designer cells are now user assignable
• All Alchemy presets now include names for Transform Pad snapshots
• 3 new traditional Chinese instrument Patches for pipa, erhu and percussion
• Over 300 new Chinese instrument Apple Loops
• Ability to enable click zones for Take Folders to provide simultaneous access to editing and Quick Swipe Comping
• Multiple additional enhancement and stability improvements

Other recent Improvements:
• New multi-threading feature improves performance when processing multiple live channels
• Faster playback responsiveness when previewing Apple Loops and performing region edits
• Flex Pitch editing can be performed within the Tracks area using a Piano Roll interface
• A collection of 30 plug-ins have been redesigned to add Retina support and improve usability
• Numerous improvements to Alchemy including Apple Loops support and 11 new spectral effects
• Additional content can be downloaded directly from the Sound Library and Apple Loops browsers
• Drum Machine Designer supports drag and drop of multiple audio files