> But I wonder how common the knowledge is these days ...

In one sense, it is less common, as you imply (though perhaps it's more that the number of high-level programmers have ballooned, rather than that the low-level ones have shrunk).

In another sense, it's more accessible than ever, with tools like godbolt[1][2], VMs, cool profilers that show you a heatmap overlaid on assembly instructions, etc.

And embedded development, where those details matter more, is still going strong, with IoT devices and so forth.

[1] part of a presentation on it, if you're not already familiar: https://www.youtube.com/watch?v=kIoZDUd5DKw&t=1191s [2] the site itself: https://godbolt.org/