I don't think that's not the reason at all, features are regularly added to ruby with much more complicated consequences for the parser. I would speculate the

I don't think that's not the reason at all, features are regularly added to ruby with much more complicated consequences for the parser. I would speculate the reason is that it's a very consequential change for the user, and they don't want to commit to it yet. I bet Ruby 4.0 will have optional inline types, if it turns out RBS are as functional as is expected.