That's oversimplifying it a bit.

Yes, at a basic level they are still basically a hollow metal ball full of explosive and triggered by a timefuse, just as they were back in WW2, in this sense they haven't really been redesigned.

But that's like saying Fords original cars from the early 1900s are the same as modern cars - they have four wheels and are powered by gasoline fuelled engines - and haven't really been redesigned.

No country (that I am aware) of still uses the same grenades they used in WW2. In fact, there were many different designs of grenades during WW2 and many different designs of grenades nowadays.

Things such as improving how the shell fragments by altering its internal structure (see here whereas in contrast to early ww2 were just rough cast iron). Improving safety features such as different styles of pins/redundant clips (see same link). Probably also changes to how the triggering mechanisms were done, such as (I assume, maybe) making them more reliable.

Then there are different kinds of grenades for different purposes - rifle grenades, offensive grenades, defensive grenades... designed differently for different purposes.

There has definitely been redesigns since the original grenades. Like with anything else in this world, few inventions remain unchanged and unimproved over time.