I want to learn Go.
From what I understand, it’s not default because there are performance regressions in some edge cases. But the typical performance is much better and the API is also much better.
To anyone writing new code that does anything with JSON—turn it on. Use it. I think it will be on by default soon enough anyway (1.27).
Go remains my favorite language for the tooling alone (while not forgetting about so many other great features)