I agree that sbrk/brk are nearly deprecated. But not all allocators I studied use mprotect or mmap. Most reference material (old OS books & blogs) still refer to sbrk/brk and/or malloc when discussing reference counters or garbage collectors. And Linux has a nice gotcha (see the Linux notes here[1]) in its implementation of the sys_brk syscall. It matches neither of the interfaces described for sbrk or brk exactly that most people are familiar with.
For me, however, I have a personal goal to show how to implement the basic newlib porting requirements [2].