`_` being assigned the result of the last statement evaluated is an IRB feature [1], not a ruby feature - hence the note that it is not a universal solution.
Why does this need to be in the language when both major REPLs already support it? Why must this be yet another alternative way to accomplish the same thing as the way that’s existed since the first days of the language?
There is no argument in favor of this addition. It doesn’t allow anything new, it provides a pointless alternative to the current way of doing things, and the one situation where it’s useful—REPLs—there’s already a feature that renders it completely unnecessary.
[1]: https://github.com/ruby/irb/blob/master/lib/irb/context.rb#L...