We did use your standard library annotations for Sorbet and have many fixes to them sitting in your pull request queue: https://github.com/plum-umd/rdl/pull/68 https://github.com/plum-umd/rdl/pull/72 https://github.com/plum-umd/rdl/pull/57 . We stopped submitting more since these weren't being upstreamed.
Before we started our project, I evaluated rolling out RDL instead of building our own typechecker. Trust me, I would MUCH rather use an existing project than have to build our own, but sadly it just didn't scale to our millions of lines of code.
I'm more than happy to chat about the details of why we didn't use RDL if you'd like to email me at sorbet@stripe.com. Thanks again for your great project, we're standing on the shoulders of giants.