This commit (in 7.4.13rc1) “added freezing of all default prototypes to JS initialization”.

The Upgrade notes for 7.4.13 say “built-in object methods are now read-only”.
I believe it should actually say “built-in objects are now read-only”.
At least, that was my experience. I had the following in a pre-processing script:

data = JSON.parse(value);

// Polyfill for padEnd
if (!String.prototype.padEnd) {
  String.prototype.padEnd = function (targetLength, padString) {
    padString = padString || ' ';
    var str = String(this);
    while (str.length < targetLength) {
      str += padString;
    }
    return str;
  };
}

...

and after the upgrade, it started failing:

TypeError: undefined not callable (property 'padEnd' of 'host_name')
at [anon] (duktape.c:69017) internal
at [anon] (function:27) preventsyield

So I was unable to extend the String.prototype object - ie String.prototype became read-only.
Converting the polyfill to a standalone function resolved issue.