This is a combined changelog including all of the betas since the 1.5.9 release. For specifically beta 5 to release, check the tag compare.

• Rewrote the deletion process
  • Prioritizes deleting entirely orphaned or unvisited regions, yielding a higher success rate and greater reduction in world size
  • Focus on providing data on-demand to reduce memory footprint
  • Chunk's last modified timestamp is now used to determine visit status in addition to flags, guarantees chunks that see regular use will be considered visited
  • Minimized main thread impact - only hooks for plugins that are not capable of (safe) asynchronous operation are queried on the main thread, everything else is handled async
• Minimized interaction with flagging system on main thread
• Converted flag file saves to SQLite (Thanks @Hdadaw for the initial implementation)
• Optimized flag storage system
  • Reduced duplicate saves
  • Queued batches of database operations to prevent too many simultaneous queries
• Added more info to /regionerator check
• Added /regionerator cache for checking cache/queue counts. This is only useful for users looking to see how conversion from YAML to SQLite is going.
• Added tab completion to commands
• Fixed Regionerator running cycles if dependencies were present but nonfunctional
• Fixed chunks/regions in nether/end worlds not being deleted
• Improved hook performance where possible
• Added hook for Civs
• Added hook for GriefDefender
• Removed hook for ASkyBlock - ASkyBlock has been replaced by BSkyBlock, and BentoBox's island deletion system essentially removes the need for Regionerator present.
• Exposed API for deleting chunks
• Removed events fired when Regionerator attempts to delete area - Regionerator makes a best-effort attempt to delete area, and deletion is not guaranteed to happen. Developers who are looking to support Regionerator deleting area in which they have data stored should instead erase data when the ChunkPopulateEvent is called.

Standard disclaimer - if I don't use a protection plugin I don't test the hook. /regionerator check is your friend.