Unfortunately, the alternative they recommend (Cleaner) only exists since Java 9, so it cannot be used by libraries that wish to remain compatible with Java 8 (which, in my experience, is most or all of them; so far, I haven't seen any Java library which has dropped compatibility with Java 8, and many only recently dropped compatibility with Java 6 or 7).
The actual alternative is PhantomReference, which exists since ancient times. Cleaner simply provides safe way to use PhantomReferences, which are rather tricky to use (by Java standards).
