And it being done in a "bugfix" release? That's unacceptable.
Conversely, for people new or somewhat-new to the language, the change probably makes sense from a principle of least surprise. From the start, you're taught that Strings are immutable objects, so you probably understand that `.substring()` produces a new instance object. Not having the original memory freed when you remove all references to the original string would likely be puzzling at first.
In this respect, the Java/Oracle folks likely decided that optimizing for the "parsing/tokenization" use case (where you make lots of substrings from a large original string and thus it makes sense to use the same underlying character array) was more novel and less frequent than the use case of "just pulling a small substring from a much larger one and then discarding the large one."
You can roll your own for the standard String too, through the bootstrap class loader.
Although I don't know how many assumptions about the internals of the string class are baked into the JVM. But I think you could replace substring() relatively safely.