I'm assuming you're talking about the "managed" pattern you'd see in stuff like Hashmaps and ArrayLists. To my knowledge that style is falling out of favor, and I think the managed versions will be removed from the standard library.

I haven't seen anyone use a global allocator in the way you're talking about, and if you did I feel like it goes directly against the Zig ethos. Part of the benefit of allocators being passed around is that all allocation are explicit.