I have, and speaking as a guy who spent a big chunk of his career working with large teams on large code bases, I think private and const are small potatoes next to really big domain-specific semantic constraints. So to me, providing small value in exchange for making other things harder to do is not a win.

It feels like we're circling around a local maximum and nobody is trying to make a really big leap when thinking about JS.next. We're getting the warmed-over leavings of middle-of-the-road language ideas.

If you really want constraints that are tailored for a team environment, make them programmable and put const and private in the standard lib. Then let Alice write something that prevents Bob from putting text in a SQL query or opening up an XSS vector in raw HTML. Or let Carol write something that ensures that all code makng certain writes to the database have an authorization check baked in.