Mine.js

// generators are lazy so maxItems isn't needed anymore - you just take as many items as you need
// it's also lazy, so if you need less items you will pay less for it. Naming a Set `set` is like naming a Number `number`
const uniqueMergeWithoutMax = function *(arrays, withoutValue) {
  const seen = new Set();
  for(const array of arrays) {
    for(const item of array) {
      if (item === withoutValue || seen.has(item)) continue;
      seen.add(item);
      yield item;
    }
  }
};

Yours.js

const uniqueMergeWithoutMax = (arrays, withoutValue, maxItems) => {
  const set = new Set();
  const res = [];
  const iterator = (item) => {
    if (item !== withoutValue && !set.has(item)) {
      set.add(item);
      res.push(item);
    }
  };
  for (let arraysIndex = 0, arraysLength = arrays.length; arraysIndex < arraysLength; arraysIndex++) {
    const currentArray = arrays[arraysIndex];
    for (let index = 0, currentArrayLength = currentArray.length; index < currentArrayLength; index++) {
      const item = currentArray[index];
      if (res.length === maxItems) {
        return res;
      }
      iterator(item);
    }
  }

  return res;
};