Yes, my first reaction is not to add a new keyword. Also to be honest, this seems to be a rare requirement - never had this ask before, I guess API testing would generally mean predictable responses.
My proposal is that you can write a custom function:
* def statusSuccess = function(){ var status = karate.get('responseStatus'); return status >= 200 && status < 300 }
* url 'https://httpbin.org'
* path 'status', 200
* method get
* assert statusSuccess()
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