Adding manual arena memory management seems like a weird choice for a supposedly memory-safe and GC'd language where this is supposed to be an implementation issue. I mean, you should hear the screams from Rust/Go people about how bad C is, and now you're just going to do it?
Adding more unsafe tools to Go or Rust do not diminish their safety guarantees. In fact, when Go and Rust add new unsafe features, they generally do so in a fashion that is much more constrained and has better defined edges than traditional methods, so that it's easier to use them correctly and easier to detect misuses. For example, the borrow checker still runs in Rust unsafe blocks; you still have to circumvent the borrow checker even once you're in unsafe. As for this arena package, it seems to have a few safety mechanisms, and hell, I don't even think it's off the table that it could be made entirely memory safe. This is especially true since memory safe does not mean "free of runtime errors", so all that is needed is to be able to detect the error conditions without allowing potentially undefined behavior to occur first.
Programming language experiments like Go's new arena package are good; they allow exploring what you can do to improve old concepts.