SLUB became the default memory allocator in the Linux kernel starting with version 2.6.23.
SLUB is a kernel allocator designed to efficiently manage small, frequently created objects such as internal data structures. Instead of requesting a fresh physical memory region for every object, it organizes memory into caches associated with particular object types. Freed objects can then be reused quickly, reducing allocation overhead and helping control fragmentation.
Linux kernels of that era offered several slab allocators. SLAB was the older and widely used implementation, while SLOB was a smaller design aimed at systems with limited memory. SLUB was selected as the default because it simplified some internal bookkeeping and offered good scalability, especially on modern multiprocessor systems. The choice did not mean SLAB and SLOB immediately disappeared; they remained selectable alternatives for years.
The version number is an important part of the answer: Linux 2.6.23 made SLUB the default, not merely introduced it. Later kernel development removed SLOB and eventually SLAB, leaving SLUB as the only remaining slab allocator in current Linux kernel lines.