In this project, simple: we don't have to. Despite being a decade old application, it was entirely AJAX-driven and front-end focused even back in 2010, and IE support was dropped entirely a few years ago by the business. Works in Edge just fine, as to be expected

Makes sense. I think the two options are just dropping IE support entirely or using (usually third-party) polyfills.

Many of us do not need to deal with IE. I haven't even needed to deal with mobile in years. Depends on requirements and use case.

Agreed. But the reason I asked because I am assuming they are making changes to a long-standing project that previously relied on third-party libraries (for browser compat?). I'm interested in that specific situation, not all projects in general.