There was never a proposal to automate arenas in Go code, and that wouldn't even make sense: the point of arenas is that you bump-allocate until some program-specific point where you free all at once (that's why they're so great for compiler code, where you do passes over translation units and can just do the memory accounting at each major step).

(Yes: I used arenas a lot when I was shipping C code; they're a very easy way to get big speed boosts out of code that does a lot of malloc).

> How many things would you have to "just" rewrite?

The same ones you'd have to rewrite using the (experimental) arenas implementation found in the standard library. While not the only reason, this is the primary reason for why it was abandoned; it didn't really fit into the ecosystem the way users would expect — you included, apparently.

"Memory regions" is the successor, which is trying to tackle the concerns you have. Work on it is ongoing.