Async vs. non-async is the main example today. There are libraries that support one or the other, or sometimes one library will have two usage modes (effectively two different code bases) because you can't really mix them.
In the future who knows, because we don't know what features will get added to the language.
std vs nostd is another big one. Within nostd there are a ton of tiny fragmented worlds. For example, the Linux kernel ecosystem will likely development its own flavor of rust, especially when it comes to memory model. Old Linux distributions will end up with fairly ancient compiler versions that require code to stick to older conventions. I doubt well end up with people stuck on targeting c89 sort of situations, but things may trend in that general direction.
If you develop that for servers or mobile/desktop applications it might look more homegenous, but their are a lot of segments beyond those out there.