In fact, prior to Linux 5.5, Linux had a direct sysctl syscall and removed it — which, uh, is clearly an ABI break.
With Linux's sysctl pseudo-fs model, you can argue the actual structure and behavior is just some aspect of sysfs, and the open/read/write syscalls are obviously not broken, but I think that's pretty simplistic. sysctls (and sysfs) are provided by the kernel.
Linux doesn't break ABIs that Linus judges to be worth more stable than rototilled. That's all. Usually Linus swings conservative on this.
In fact, prior to Linux 5.5, Linux had a direct sysctl syscall and removed it — which, uh, is clearly an ABI break.
With Linux's sysctl pseudo-fs model, you can argue the actual structure and behavior is just some aspect of sysfs, and the open/read/write syscalls are obviously not broken, but I think that's pretty simplistic. sysctls (and sysfs) are provided by the kernel.
Linux doesn't break ABIs that Linus judges to be worth more stable than rototilled. That's all. Usually Linus swings conservative on this.