hsbt (Hiroshi SHIBATA) and I have been working to make RBS one of the bundled gems.
We are finalizing the patch and hope we can finish it tonight.
I'd like to ask Matz to approve bundling it in Ruby 3.0 (hopefully before preview 1).
RBS is a Ruby gem to support static analyzers in Ruby 3. It consists of the standard library type signatures (.RBS files) and Ruby code to parse and process .RBS files. The repository for RBS is https://github.com/ruby/rbs.