[twtw]

Can something be obsolete if it is still extremely widely used?

IIRC, Linux and FreeBSD both use a buddy allocator for physical memory, and I'm pretty sure it is used (in addition to other allocators) in jemalloc as well.

[pcwalton]

I don't think jemalloc uses any buddy allocators. It's a segregated free list allocator, like most C mallocs and tenured generation allocators.