First, this extends deep into the core of Julia, and I don't see how a traits package would be involved with that.
Second, and this dovetails with the first issue, this needs to be something that people actually use as a default action. Part of that means ensuring the built-in types make use of this.
Related to all of this, I worry it's too late to really make a meaningful change here. The culture and existing packages are already set without it. Adding the feature as a requirement isn't going to happen anytime soon unless people are willing to continue to break things post 1.0. And it needs to be a requirement or it won't get used except by people that will already provide documentation.
The lack of interest I mentioned mainly came from some github issues which either received little attention, had creators that had a very "maybe it would be nice if" attitude, and responses that were questioning the benefit compared to the cost of implementing the syntax changes.