Yes it was a nice/quick/dirty way to persist an object graph, I bet it's been abused richly in a lot of codebases out there, it'll be curious to see how this is handled in terms of breakage i.e. the JDK that introduces this might be the Python 2/3 showdown (if that's still a thing).
Be curious to do a Github wide grep for ObjectOutputStream or something similar and see what it's like in open source land.
This is made more exciting by the fact that Java rarely introduced big breaking changes in the past. Especially deprecated classes (java.util.Date...) never were actually removed. Starting this now is going to wreak havoc.
We're already starting to see the effects in Java 9. Our team attempted to use it on our current project and many of our dependencies broke because they were using deprecated parts of the API that were removed. We ended up having to drop back to Java 8.
Be curious to do a Github wide grep for ObjectOutputStream or something similar and see what it's like in open source land.