Should I use the final modifier when declaring case classes?

It is not redundant in the sense that using it does change things. As one would expect, you cannot extend a final case class, but you can extend a non-final one. Why does wartremover suggest that case classes should be final? Well, because extending them isn't really a very good idea. Consider this:

scala> case class Foo(v:Int)
defined class Foo

scala> class Bar(v: Int, val x: Int) extends Foo(v)
defined class Bar

scala> new Bar(1, 1) == new Bar(1, 1)
res25: Boolean = true

scala> new Bar(1, 1) == new Bar(1, 2)
res26: Boolean = true
// ????

Really? Bar(1,1) equals Bar(1,2)? This is unexpected. But wait, there is more:

scala> new Bar(1,1) == Foo(1)
res27: Boolean = true

scala> class Baz(v: Int) extends Foo(v)
defined class Baz

scala> new Baz(1) == new Bar(1,1)
res29: Boolean = true //???

scala> println (new Bar(1,1))
Foo(1) // ???

scala> new Bar(1,2).copy()
res49: Foo = Foo(1) // ???

A copy of Bar has type Foo? Can this be right?

Surely, we can fix this by overriding the .equals (and .hashCode, and .toString, and .unapply, and .copy, and also, possibly, .productIterator, .productArity, .productElement etc.) method on Bar and Baz. But "out of the box", any class that extends a case class would be broken.

This is the reason, you can no longer extend a case class by another case class, it has been forbidden since, I think scala 2.11. Extending a case class by a non-case class is still allowed, but, at least, in wartremover's opinion isn't really a good idea.