At this point you'd need a 10 year old never-updated browser to run into this issue. Which, sure, maybe you need to cater to people who use browsers 10 years out of date. I don't think it's a huge market frankly and 10 year old browsers have larger issues like security.
I mean, JavaScript doesn't just run in browsers. Some older forms of JavaScript run in all kinds of software--embedded in Windows (JScript), embedded JavaScript engines in other runtimes (Java, Qt Quick), older Node.js software that people are stuck on for whatever reason. I'm sure many people don't care about making all of those devices obsolete but maybe with a slight amount of effort we can try to slow down the inevitable filling of landmines with perfectly workable devices?
Okay, those cases seem like situations where you'd need to be explicitly targeting those devices. Like if you're shipping new code to users that is being run on an old runtime, then sure, you should make sure that code is legacy JS. I don't see how that is unreasonable?
