There is an official spec here: http://rethinkdb.com/docs/writing-drivers/ Not quite TS, but it's well defined and new releases of the spec are carefully managed.

For me the TS is a tool to ensure my code is not using deprecated API. This is a partly reason why Facebook is also pushing typing to JS with Flow.

Edit: And Guido is pushing it to Python with PEP 484: https://www.python.org/dev/peps/pep-0484/

It's inherent problem with dynamic languages, you have to read all new release documents and migrate your code. With typed code I at least can be somewhat sure I'm not using deprecated calls and such just by compiling.