Stripe has built a type checker for Ruby that they're using internally. I don't think it has been released yet, but you can get a preview of it at https://sorbet.run/.
We had a discussion about Ruby's type annotations 4 years ago at https://news.ycombinator.com/item?id=9481186
I found it by googling the status of the work on Ruby 3 type annotations. The top comment is mine and I still agree with the past myself.