It was a main focus of Fortress (https://en.wikipedia.org/wiki/Fortress_(programming_language...), for example, which was interesting but never quite got off the ground in the same way as other competitors, in part because of the focus on notation implementation.

If anything, my experience following Fortress led me to believe what Steele was discussing in his talk is both a tremendous opportunity but also much more difficult than the impression he creates.

For what it's worth, I thought the talk was really interesting and I would like to see it taken more seriously, if not as an immediate goal, then a long-term one.