With Android that's definitely not the case. Supporting older phones might get harder over time because you can't use any new APIs introduced in more recent OS releases, respectively always have to provide some fallback code path, and occasionally (at least if you want to publish on the Play Store) you're forced to use the new APIs, so you can't avoid the complexity of supporting both old and new APIs.
Plus if you're using any dependencies, you're also bound by whatever minimum API version all your dependencies are using. (Even Google's support library – on the one hand that one does try to somewhat smooth over the API differences between various OS releases and make your life easier, but eventually it'll also drop old Android API versions – on the conservative side, but eventually it'll do.)
But – if you're prepared to somehow work around all that, there's no hard cut-off, and a modern Android toolchain will still happily produce APKs that would run on by now very old phones, too.
Like e.g. I've taken an old app that had initially been developed during the Android 2.x era (around 2010/11) in order to fix a few annoyances and add some features. Since I didn't do any kind of radical overhaul of the original code so far, the resulting app happily runs both on modern phones (albeit with a somewhat older look-and-feel), but also on the oldest emulator image I could still get to work on my computer (Android 2.3.3 / API10 from February 2011).
Plus if you're using any dependencies, you're also bound by whatever minimum API version all your dependencies are using. (Even Google's support library – on the one hand that one does try to somewhat smooth over the API differences between various OS releases and make your life easier, but eventually it'll also drop old Android API versions – on the conservative side, but eventually it'll do.)
But – if you're prepared to somehow work around all that, there's no hard cut-off, and a modern Android toolchain will still happily produce APKs that would run on by now very old phones, too.
Like e.g. I've taken an old app that had initially been developed during the Android 2.x era (around 2010/11) in order to fix a few annoyances and add some features. Since I didn't do any kind of radical overhaul of the original code so far, the resulting app happily runs both on modern phones (albeit with a somewhat older look-and-feel), but also on the oldest emulator image I could still get to work on my computer (Android 2.3.3 / API10 from February 2011).