This is a meta-issue checklist to keep tracks of the issues that ought to be resolved before branching OCaml 5.0 .
(for maintainers: please feel free to edit this post to update the status (or descriptions) of the various items on this list.)

Platforms

32-bits:

• bytecode-only [works since Assorted fixes found while restarting the Jenkins CI #11037]
  (patches accepted for word-size independence)

Arm 64

• Arm64 multicore support #10972 now merged. Also added CFI directives to runtime/arm64.S to support stack backtraces by 3rd-party tools (ARM64 CFI support #11099).
  - [ ] technical documentation for the memory model (updated PLDI paper with object publication)
  (see also OCaml multicore memory model and C (runtime, FFI, VM) #10992, Wrong memory order constraint in caml_modify #10886) (to be done later)
• litmus tests (Memory model tests #11004)
• high-level descriptions of the assembly functions in runtime/$ARCH.S (Documentation improvements for native stack switching functions #11059)

Windows

• mingw-64 with winpthreads

Middle-end

• flambda: check optimisation safety

Tooling

• ocamldebug: assess feasibility ( Restore basic functionality to the bytecode debugger #11065 )
• event ring (Ring-buffer based runtime tracing (eventring) #10964) to replace eventlog

Runtime

• limit virtual memory reservation (Means of limiting how much memory is being reserved by the runtime, so that Valgrind and AFL can be used #10971) [temporary workaround introduced by Assorted fixes found while restarting the Jenkins CI #11037] - follow-up API work in Gc.set now changes the minor heap size of all domains #11142
• bytecode performance assessment (see Bytecode compiler emits too many calls to caml_ensure_stack_capacity, causing a slowdown #11062)

Stdlib

• full audit: Audit stdlib for mutable state #10960
• documentation: concurrency safety (Handle alerts in ocamldoc #11024, Concurrency safety documentation, part one: concurrency unsafe types #11193, stdlib concurrency safety documentation, part two: global states #11227)

Documentation

• API: Effects, Domains, Atomic
• manual: Effects (Effects manual #11093)
• manual: Domains (Manual chapters on parallelism and memory model #11280)
• manual: memory models (Manual chapters on parallelism and memory model #11280)

CI

• Thread Sanitizer (see ThreadSanitizer issues #11040) (removed, hopeless)