It should be opt-in though. A compiler-flag for debug builds or similar.
I want to be able to design objects that have toString() and I want to be able to design objects that don't have toString().
The same goes for clone(), hash(), getPtr(), getEndian(), equals(), toBytes(), toJson(), encrypt(), delete(), isNumeric(), toXml(), serializationVersion() or compare().
Now I'm wondering... are there languages which formalize "implement with a throw" into some kind of explicit refusal to implement a method? Obviously there are method-sharing approaches other than inheritance, but I've never heard of "you must implement this, or explicitly choose not to".
Java doesn't have a language-level exception that's specifically for unusable methods, which can be a bit awkward. UnsupportedOperationException is the recommended answer, but it's relatively nonspecific as to why your operation wasn't supported. Apache's Commons extends that with NotImplementedException for cases where the call isn't definitionally impossible (e.g. adding to an unmodifiable map), but no implementation exists.