Re: [Proposal] define String#json? to return boolean value if the object is json string
Note that I'm not on the core team, so my answer is far from final, but: just a heads up that the current refrain on changes to Active Support (and particularly the core extensions) is roughly "We avoid adding new things to Active Support unless they are used in the framework or by a large number of applications, so we need to know why you think this feature is useful to take any decision" — example.
You could open a PR with your change to get more discussion since it's a reasonably minimal and understandable diff, but expect pushback unless you have a particularly compelling use case. Since this is a core extension update to String, you could also lobby to have it included in Ruby directly instead.