https://www.ruby-lang.org/en/news/2020/12/20/ruby-3-0-0-rc1-released/

I've been working with Ruby for the past ~4 years and have felt the pain of a (somewhat) missing type-system. To me RBS feels like a crutch and is not going to "fix" the problems Ruby's type-system has.

Ruby doesn't really have a type-system: Any thread in a Ruby process can just monkey-patch and override all types, pulling away the rug under your feet.

I believe that Crystal had the right idea at the right time: Rebuild the whole thing and do it right this time.