All in all, Java is pretty unique in the level of backwards compatibility it provides, I don't think any other language is comparable to this level. Especially that it is both source and binary compatibility.

While this distinction is often useful, here we have to think about it from the perspective of users: you press the button to upgrade your toolchain, and code that formerly worked stops working. If a language supported upgrading your compiler/interpreter separately from your standard library then that would be different, but generally a standard library version is considered tightly coupled to a language version.

But the platform itself is extremely backwards compatible, you can find some old jar file created for a university course that still runs without an issue. Of course if you have a bunch of libraries that may so stuff like touch internal details, you lose some of that compatibility (sun.misc.unsafe package can access some internal details, memory). Recently even this latter has been locked down more and more, so maybe that's one reason for your experience (e.g. previously one could set a private field to public and access it, now you can only do that if you explicitly give a flag)

And given that Minecraft was (is?) proprietary, reverse engineered code base where plugins were hacked into, I guess this brittleness makes sense.