Memory map of the Apple II ROMs

Apple II guy here. SWEET 16 was not used that much. The Apple II+ -- introduced 2 years after the original Apple II -- used up the whole ROM space for the BASIC interpreter, so there was no longer room for extra stuff like SWEET 16. The original Apple II only sold ~40,000 units, so the Apple II+ memory map became the standard one.

OTOH, it is true that, pretty early on, it became common for the ROM space to be switchable. Instead of reading the on-board ROM, you could have the system read from ROM (or, with the "Language Card", RAM) on a plug-in card. That means that a typical Apple II-whatever in the 80s would actually have had SWEET 16 available, although somewhat hidden, not well known, and without readily available documentation.

Still, I did a lot of low-level programming, and I never used SWEET 16. Nor did I ever run across any code that did, except stuff sold by Apple, like the Programmer's Aid #1.