Apple developed most of it internally, proposing collaboration with Kronos in 2008, and released their implementation in 2009 with Snow Leopard. Apple supported it in their software for nearly a decade, only discontinuing it with Mojave and iOS 12 in 2018, a year after they introduced Metal v2.
It's still an open standard managed by Kronos and according to Wikipedia, has conforming implementations by Altera, AMD, ARM, Creative, IBM, Imagination, Intel, Nvidia, Qualcomm, Samsung, Vivante, Xilinx, and ZiiLABS. Kronos released a new major version of the specification, OpenCL 3.0, in September 2020, so the standard has clearly not suffered from the departure of Apple's implementation.
While Apple no longer supports OpenGL and OpenCL in their operating systems, Apple continues to be a "promoter" level member of the Kronos Group, and they continue to be involved in supporting Kronos' specifications and implementations through some of their other open source projects (e.g. Clang and LLVM).
Apple created, open sourced, and donated OpenCL to Khronos as an open alternative to Nvidia's proprietary CUDA, for example.