Ada does. It has been through 5 editions so far and backwards compatibility is always maintained except for some small things that are documented and usually easy to update.

I'd normally be inclined to agree that minor things are probably good enough, but "absolute non-negotiable" is a rather strong wording and i think small things technically violate a facial reading, at least.

On the other hand, I did find what I think are the relevant docs [0] while looking more into things, so I got to learn something!

[0]: https://docs.adacore.com/gnat_rm-docs/html/gnat_rm/gnat_rm/c...

> except for some small things that are documented

I can't think of any established language that doesn't fit that exact criteria.

The last major language breakage I'm aware of was either the .Net 2 to 3 or Python 2 to 3 changes (not sure which came first). Otherwise, pretty much every language that makes a break will make it in a small fashion that's well documented.