EOL or no, there are many people who still use older OSes because they own older computers, can't afford an upgrade or don't want one (it works just fine!), and can't be bothered with figuring out running a newer OS on officially unsupported hardware.
Case in point: I built a macOS app that implements Google's Nearby/Quick Share (an AirDrop-style file sharing thing on Android). Multiple people tried running it on Catalina and were disappointed that it wanted a newer OS. So I did end up backporting it to Catalina.