I'd bet that 90% of Javascript code probably still runs today, but that's only because I think that so much JS has been written in recent years compared to the

I'd bet that 90% of Javascript code probably still runs today, but that's only because I think that so much JS has been written in recent years compared to the early years. If you take the average codebase from, let's say 2006, I would honestly put down money that under 25% would still run to this day. While most churn that people complain about on the frontend now is "optional" churn, the first two decades were full of breakage. Not only are there wide swaths of APIs that have been rightly removed, there are even keywords that have been deprecated and aren't available in strict mode.