.preventDefault() not working in on.('input', function{})

To answer your main question:
.preventDefault() cancel behavior of the events that are cancelable. input event is not .

To check whether an event is cancelable, try: console.log(event.cancelable);.
For input it will say "false"

You may want something like this (untested):

const limit = 10;var rem = limit - $('#About').text().length;$("#counter").append("You have <strong>" + rem + "</strong> chars left.");$("#About").on('input', function(event) {    var char = $(this).text().length;    rem = limit - char;    if(rem <= 0){        $(this).text($(this).text().slice(0, limit));    }    $("#counter").html("You have <strong>" + rem + "</strong> chars left.");});

EDIT

JSFiddle demo
Since it's contentEditable, I had to use an additional code (from this answer) to set a caret at the end of the input.

use keydown instead of input

jQuery(document).ready(function($){    const limit = 10;    rem = limit - $('#About').text().length;    $("#counter").append("You have <strong>" + rem + "</strong> chars left.");    $("#About").on('keypress', function(event) {        var char = $('#About').text().length;        if(char>limit)        {            return false;            //or            event.preventDefault();            //TODO        }        rem = limit - char;        $("#counter").html("You have <strong>" + rem + "</strong> chars left.");    });});

DEMO

el.addEventListener('input', function(event) { if ( this.innerHTML.length >300  && event.keyCode != 8) {   document.execCommand("undo");   } });