I know that other independent browsers that I used to use back in the day just gave up because the pace of divergence pushed by the major implementations meant that it wasn't feasible to keep up independently.
I still miss Konqueror.
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.
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.