Mainlined where? There is no mainline for allocators, C and C++ rely on the system-provided allocator as their baseline, possibly replaced by an alternative / custom allocator (though the same allocator could be system-provided in one place and custom on another e.g. jemalloc is the system allocator for freebsd but commonly used as custom allocator on other platforms), supplemented with bespoke allocators for task-specific schemes (e.g. arena or linear allocators where that makes sense).
You could try to get the mesh methodology integrated into existing allocators I guess, but there is no such thing as a "mainline allocator".
e.g. jemalloc is the system allocator for freebsd but commonly used as custom allocator on other platforms
I think they just mean mainlined on a particular platform. If FreeBSD were to switch its system allocator from jemalloc to mesh, for example, I’d count that as mainlining.
You could try to get the mesh methodology integrated into existing allocators I guess, but there is no such thing as a "mainline allocator".