It's already gaining adoption outside of the BSDs. OS X/iOS seem to have it as part of their libmalloc. Android Bionic libc has it as part of the code they sync from upstream OpenBSD.
Many open source projects include their own, or simply bundle the OpenBSD implementation:
* mandoc
* flex
* unbound and nsd
* tor
* tmux
* libbsd
* libressl
* xorg-xserver
* ...
The list only continues to grow, several more examples to be found on GitHub.
Darwin (macOS / iOS) is often counted as "one of the BSDs", just a fairly weird one. Big chunks of the standard library are copied from FreeBSD with changes to work on top of Mach.
There's code for it in Darwin's libmalloc, but it's not exposed as API.
reallocarray() has some difficulties as an interface, mostly inherited from realloc(). I'm a bigger fan of reallocarr(), but that's NetBSD only. We (the operating systems community) need to find a consensus here, but I'm not convinced that reallocarray() is that consensus yet.
Many open source projects include their own, or simply bundle the OpenBSD implementation:
The list only continues to grow, several more examples to be found on GitHub.