Can't say I've heard of those, but TeXmacs does pride itself on being structured.

Those exist since the 1990's, and sorry to say, they are hardly unknown.

Perhaps they aren't. TeXmacs is also from the 1990s and from your brief description, no compelling reason was offered to prefer them. :p

So here is one, plenty of people used those at CERN, I haven't ever seen TeXmacs in action.