use push instead of pushObject , javascript arrays does not have pushObject method,
when you push objects to the array , @tracked doesn’t deep track objects , for @tracked todos still the same , so assign it an new array to make it re-render the content on the hbs this.todos = [...this.todos];
you are welcome, https://rfcs.emberjs.com/id/0848-deprecate-array-prototype-extensions this is good to read to understand why you get the error, ( older version of embers extends by default ember mutable arrays that has pushObject , and in recent Ember versions this was deprecated )