You are confusing backwards and forwards compatibility. Those sites may have added features that your iPad does not support, which is why it broke, if they have not added those, it might still work.

However JS is not 100% backwards compatible either, it is in many cases, largely backwards compatible, but there are rare cases of bug fixes, or deprecated APIs that might be removed and break old code, but this is not even JS itself, it's more like web/engine standards.

You are talking about forward compatibility.

JS is backwards compatible: new engines support code using old features.

JS is not forward compatible: old engines don't support code using new features.

Regarding your iPad woes, the problem is not the engine but websites breaking compat with it.

The distinction matters as it means that once a website is published it will keep working. The only way to break an existing website is to publish a new version usually. The XSLT situation is note-worthy as it's an exception to this rule.