alt Hacker News> But if the code already exists and is working, is it really that costly to just not break it?
It depends on the feature, but in many cases the answer is in fact 'yes.' There's a reason why Alpha support (defunct for decades) still goes on but Itanium support (defunct for years) has thoroughly been ripped out of systems.
An interesting example of this was i386 support (as in being able to run on the 80386) being removed way back in kernel 3.8 (early 2013). If you can support 80486 why can't you support 80386?
Well, there really wasn't much support for atomic instructions in x86 before the introduction of compare-exchange in the 486. Any time you wanted guaranteed atomicity on a 386 you had to disable interrupts, which among other things means that if you lock up during a critical section your entire system hangs. Another implication is that nearly all of our lockless data structure constructs depend on compare-exchange instructions.
It vastly simplified some very tricky sections of the kernel to remove support for systems that don't have hardware atomic instructions, so it ended up being done.