Sadly it's a lot more than old IE versions, there are a lot of mobile devices with surprisingly old/buggy browsers around. Of course this is highly dependent on

Sadly it's a lot more than old IE versions, there are a lot of mobile devices with surprisingly old/buggy browsers around. Of course this is highly dependent on the target audience but at my last job we did some runtime feature detection (basic stuff like arrow functions, let/const...) and shipped a modern build or a legacy build. And something like 10% of users got the legacy build, most of them on mobile browsers.