They are both correct without "that".
Student or Learner
Please would you correct the mistakes in my sentences. Are these sentence correct without the world "that"?
1. The architect promised a beautiful house the world has never seen before.
2. The soldiers committed such a horrific crime humankind has never witnessed before
***** NOT A TEACHER *****
In my very humble opinion, your two sentences do not sound natural to me.
I would feel more comfortable with something like:
1. The architect promised Mr. and Mrs. Smith that he would build the most beautiful house (that) the world had ever seen.
2. The soldiers committed the most horrific crimes (that) humankind had ever witnessed.