Some of the highlights in OCaml 5.3.0 are:

• Syntax for deep effect handlers
• Restored MSVC port
• Re-introduced statistical memory profiling (statmemprof)
• utf-8 encoded Unicode source files and modest support of Unicode identifiers
• More space-efficient implementation of Dynarray
• Improved metadata on the pairs of declarations and definitions for merlin.

And a lot of incremental changes:

• Around 20 new functions in the standard library
• Many fixes and improvements in the runtime
• Improved error messages for first-class modules, functors, labeled arguments,
  type clashes.
• Numerous bug fixes