Add a changelog entry for OMP's deprecation (#1712)

Co-authored-by: Christine Rose <christinerose@users.noreply.github.com>
Co-authored-by: Thibaut Mattio <thibaut.mattio@gmail.com>

data/changelog/omp/2023-10-23-omp-deprecation.md

@@ -0,0 +1,38 @@
+---
+title: OMP Deprecation
+date: "2023-10-23"
+tags: [omp, platform]
+---
+
+After [giving space for feedback and objections by the community](https://discuss.ocaml.org/t/rfc-deprecating-ocaml-migrate-parsetree-in-favor-of-ppxlib-also-as-a-platform-tool/13240),
+we have deprecated `ocaml-migrate-parsetree` (aka OMP). It is superseded by [Ppxlib](https://github.com/ocaml-ppx/ppxlib).
+
+There are four major differences between OMP and Ppxlib, which all go hand in hand.
+
+The first major difference is in the library and therefore impacts how to write PPXs.
+With OMP, each PPX author had to choose a parsetree version to define their PPX against.
+There was no version agreement between different PPXs. With Ppxlib, each PPX author uses
+the same parsetree version.
+
+The second major difference is about compatibility with new compiler syntax. While with
+OMP, each PPX was on its own parsetree version, Ppxlib keeps them all on the version of
+the latest stably released compiler. That makes using any PPX compatible with using the
+latest compiler syntax features!
+
+The third major difference is in the philosophy of PPXs. With OMP, all PPX transformations
+were global transformations, i.e., transformations of the whole parsetree. Ppxlib has
+introduced the concept of "context-free" transformations, i.e., transformations that transform
+only one parsetree node. By restricting their scope of action, context-free PPXs are a lot
+more predictable and less dangerous! Also, Ppxlib merges all context-free PPXs into one
+parsetree pass, defining clear semantics of PPX composition.
+
+The fourth major difference is in the driver, i.e., the binary that drives the application
+of all used PPXs in a project. The Ppxlib driver is significantly more performant than the
+OMP driver used to be. That's partly because it does a lot fewer parsetree migrations and
+partly thanks to merging all context-free PPXs into one parsetree pass.
+
+As a consequence of the deprecation, OMP will be incompatible with any new compiler version.
+The first incompatible compiler version is OCaml 5.1.
+
+Thanks a lot to everyone involved in OCaml's transition from OMP to Ppxlib, for example
+by porting their PPX!