That's simplifying things a little. The 90s were a completely different time in computing, still somewhat pioneer when it came to "modern" operating systems in

That's simplifying things a little. The 90s were a completely different time in computing, still somewhat pioneer when it came to "modern" operating systems in personal computing. What came before on home computers was usually tied to the actual hardware and its implementation in a very thorough way, where way more outrageous (but at the time, widely accepted) assumptions were made. For example, what memory location to write into for direct display on the screen from your application code. A few years earlier, the absolute time that a particular instruction takes. Computers became more powerful and more diverse, we added abstractions, we abolished assumptions. And still I'm pretty sure that even in Oculus (to pick up your example, I know nothing about that), there are bound to be a great deal of assumptions in the code that cease to be valid with later versions of the products.