How to use SLAB memory allocation instead of SLUB allocation in Linux kernel? As I understand it, both SLAB and SLUB are still integrated into the kernel in stable release 5.12. Is there a way to enable SLAB allocation to run on the system instead of SLUB allocation? Or is there no “toggle” to switch between them/it doesn’t work like that?

There is no toggle in runtime: you switch between SLAB and SLUB (and if you enable EXPERT mode, SLOB) by recompiling the kernel. The option is in "General setup->Choose SLAB allocator".

I haven't found any reason to change from the default, but the answer to this previous SE question indicates that there's probably little or no reason to use SLAB.