I think the last breaking change I remember was enumerations being added, which broke any code that used the class name as a variable name. But I could be wrong; it was almost 20 years ago.

With Java 11 they hid a lot of internal functions that people had used in their code, so breaking things. But that was never really part of the public API, so strictly not a breaking change.

I remember moving to Java8 changed the iteration order of hashmaps etc, which also broke some stuff for us. But again, that was mostly our fault for relying on unspecified behavior.

With Java 11 they only hid it, you just need to pass some parameters to get it working. When it truly broke, I believe, was only on Java 21, which finally removed some clutches people had been using to work around the new limitations.

Maybe not a part of language per se but they throw out corba in java 11 and this decimated some very old libraries that used is as dependency.

Thread#stop no longer works as of at least Java 21.

True, I think since Java 17 they have been removing really obsolete stuff.. I think that's when they removed CORBA from the JDK as well (though you can still get it as a library I believe). Same with Nashorn (the JS runtime).

Personally I consider 1.4.2 the point where java got stable, and had a true IO (java.nio), along with a decent JIT (hotspot)

Phew, that was around the internet Java applets phase, if I recall it correctly?

That much it was all about JSP/XML and stuff; J2EE too. JDO came soon after too. The applets did exist and (javax.)swing was a thing - but it was far from the focus... and of course Java was still considered slow.