https://github.com/ruby/ruby/blob/v4_0_0_preview2/NEWS.md

What is interesting to me is the `Ractor.shareable_proc` changes that solved a bug for a use case I was having. And in general fixes for Ractors make them more appealing to use right now, even though they have not removed the `Experimental` flag from them. They are still missing a built-in concurrency primitive like channels or a lock free queue; I'm curious what they will settle on, Ractor::Port is nice but not enough.

https://docs.ruby-lang.org/en/master/box_md.html

   Ruby Box is designed to provide separated spaces in a Ruby process, to isolate applications and libraries.

The Ruby Ractor (Actor) interface is now completely changed to use a Ractor::Port class, mirroring IPC (inter-process communication) semantics. Ractors were added in 3.0 as a way to get around the GVL/GIL, but having N number of Ruby interpreters running in a Ruby process which would enable executing on N cores at once. For me, hot take but Ractors don't seem to offer major advantages over plain-ol' copy-on-write (COW) forking.

The one "big" feature was supposed to be namespaces, which apparently have now been renamed to Ruby::Box (https://docs.ruby-lang.org/en/master/box_md.html). From what I can glean from the Ruby issue tracker, it appears this feature has been radically descoped, primarily because it had performance impacts, but also, I think probably there are realistic concerns about fit with the existing ecosystem. Unlike Javascript/Python, Ruby has never used "modules" for code isolation--everything is loaded into the global namespace (the "global dumping ground" as I call it.) Now the Box feature is only enabled with an environment variable RUBY_BOX=1

https://bugs.ruby-lang.org/issues/21311