Like Nim, you can also disable GC in D, and you can avoid GC altogether by not allocating GC'd memory (GC is only ever triggered at allocation time), or at least by preallocating and then disabling the collector. Raw allocation is always possible, and you can "emplace" D structs and classes into unmanaged memory.
The collector itself isn't being improved as far as I know -- at least I haven't seen any initiatives mentioned recently with that goal. In fairness, I haven't been following the community activity very closely in the past few months, but I think that's accurate. Nim definitely has a technical advantage re: its GC implementation.
The bigger movement has been the "@nogc initiative", which started with adding a @nogc attribute to the language (the compiler can verify that a function tagged with @nogc, and all of its callees, do not allocate GC memory). There is an ongoing initiative to make more of the Phobos library @nogc-compliant, to take advantage of this feature. There has also been a lot of work on custom memory-allocators [1], and I think the plan is to incorporate into Phobos where it makes sense, so you can have functions which take custom allocators, have thread-local allocators, etc.
https://dlang.org/phobos/std_experimental_allocator.html
I don't speak for the community or the dev team, but I think the long-term goal is to make the GC a feature that is available when you want it, but that isn't a dependency for using the standard library. Either through custom allocators or through @nogc guarantees, you'll be able to ensure that your program's memory management is deterministic.