Hopefully there should never be a repeat of that now that they have strongly encapsulated jdk internals. My understanding is that (nearly) all of the migration headaches from 8 to 9 were caused by libraries that were using improperly using jdk internals.
Devil's advocate: anything that's possible for a downstream user to access is fair game for them to use. You can certainly mark it as internal and be explicit that you reserve the right to break it later, but if it's actually possible for users to do, it's not "improper", even if it gets broken later.
That's why they sealed those holes shut, and only allow some of them with deliberate end-user command-line flags, so that anyone wanting to go that way only has themselves to blame.
JavaEE being removed, along with a package frequently used for Base64 encoding (with no replacement until several later versions). JavaFX being separated out so it's not bundled anymore, along with removing javafxpackager (since returned as jpackage). Java Web Start being removed.
Then there were all the borderline stuff. The locations of files inside the JDK all changing, like "rt.jar" went away and a lot of tools depended on that. The concept of an installable JRE was removed entirely and along with it the whole way people were used to distributing Java apps was deprecated with no replacement until much later (and the replacement was much worse in some ways). Suddenly spamming warnings to the console if you use widely used packages (which breaks anything parsing the output).
Even just changing the version number broke a lot of stuff because code had been written to assume the convention that Java version numbers started with "1."
Then when they went to 6 month releases soon after, that broke a lot of stuff because the whole ecosystem made the design assumption that Java releases were rare (stupid stuff like using enums to represent versions, the Java guys bump the version number in .class files on every release even if nothing changes).
Then people tried to use the new module system, but that broke the world too and for little/no ROI, so eventually everyone gave up. Now the ecosystem is full of broken module metadata that's there but doesn't work, and if you try to use it and report bugs they get closed with status: "don't care".
Frankly a lot of the dust has still never settled, it was a very damaging time for the Java community. Backwards compatibility über alles bitte, and that means NOT removing widely used features that were heavily developed and advertised as the right way to do things for decades.
I only see eternal stagnation as the alternative, and surely noone wants that.
Java does have very good backwards compatibility and they make every change with that in mind, but if you are big enough, no matter what you do, someone will surely depend on some stupid thing they should have never do in the first place.
