It's not very novel. There's far better ways of solving this than allowing a random string to be embedded as aux information to a struct field. Examples: F# type providers, or OCamls PPX system for extending the language in a well defined way. Macro rewriting systems also allow for better safety in this area.

This allows you to derive a safe parser from the structural data, and you can make said parser be really strict. See e.g., Wuffs or Langsec for examples of approaches here.

I’m not disagreeing that there are better ways to solve this given how other languages have implemented theirs but considering the constraints they had at the time the Go team designed this, it allowed them to implement marshaling fairly easily and leaves it open for extensions by the community.