With the below statement you are getting the first .newDiv,

document.getElementsByClassName(".newDiv")

and correspondingly you are removing it.

You should make use of closures and remove newTxt since it is already available at time of your event handler declaration.

  deletButton.onclick = function(){
    newTxt.parentNode.remove(newTxt)
}

document.getElementsByClassName(".newDiv")

The above line of code will return the fist element with class name newDiv and that is the reason why your code deletes the first element.

So in order to delete the element that got clicked you have to select the button that got clicked and delete its parent.

deletButton.onclick = function(){
    this.parentNode.remove();
}

document.getElementsByClassName will always returns an array-like object of all child elements which have all of the given class name(s).

This does not have any connection with the row from which you chose to delete the item, this involves all the element having the specified class name.

Also element.remove invokation is wrong.

You can select the row from which the delete event was triggerd by selecting the closest element with the class name newDiv from the target element of the click event.

Working Fiddle

let add = document.querySelector(".add")
add.onclick = function () {
    let input = document.querySelector(".input").value;
    let newTask = document.querySelector(".tasks")
    let newTxt = document.createElement("div")
    newTxt.className = "newDiv"
    let newP = document.createTextNode(input)
    newP.className = "newInput"
    let deletButton = document.createElement("button")
    let deletTxt = document.createTextNode("Delet")
    deletButton.className = "deletButton"
    deletButton.appendChild(deletTxt)
    newTxt.appendChild(newP)
    newTxt.appendChild(deletButton)
    newTask.appendChild(newTxt)
    deletButton.onclick = function (e) {
        removeEle = e.target.closest('.newDiv');
        removeEle.remove()
    }
}

<div class="tasks"></div>
<input type="text" class="input">
<button class="add">Add</button>