Yes but if Java 10 (for example) introduced this annotation then the JVM10 could transform a null returned from a non-nullable annotated function to an exception. Compile time static checks could also be added to guarantee that such a method does not return null. But you could still use such a function from a previous java version albeit you would sacrifice the checks.

It is a way to introduce the benefits (as long as the library is developed properly) of non-nullable types without sacrificing retro compatibility.

Edit: I remember a bug in earlier version of Swift Cocoa binding where getting calendar events without titles would hang without possibility of recourse because of this problems.