Right, Android target levels are so different in how they behave towards applications that they're practically best treated as distinct OS's to begin with. There's really no such thing as a unified Android or iOS, unlike Windows or desktop Linux where even a program from the mid-1990s will run unmodified in the latest version of the OS.

> Android target levels are so different in how they behave towards applications that they're practically best treated as distinct OS's to begin with

You can run applications running different target levels side by side though

> desktop Linux where even a program from the mid-1990s will run unmodified in the latest version of the OS

mhm... I wish but that's not so true for Linux. Your old program will likely be missing some dynamic library or be incompatible with your current libc. Desktop Linux userspace is awfully unstable, compatibility is broken left and right, basically no one cares except the Linux kernel itself. There's a reason people jokingly say that win32, through wine, is the most stable Linux API. If you still have the source code of your program (and the linux ecosystem is full of free software so that's likely), you can always recompile but you'll probably need to edit the code so it's compatible with the current versions of libraries).

I've heard macOS is not great at this neither.