I'm sure the usual suspects (Synopsys, Broadcom, ARM, Xilinx, etc.) would be happy to license something. But from what I can see all the big players make their own. I guess they're easy enough to implement yourself (as a big player) and important enough to not want to leave it in the hands of a third party.

There are also likely opportunities for additional efficiencies when you make a custom {en,de}coder for your system. I suspect (but haven't confirmed) that the typical Intel/AMD/Nvidia/Apple multi-function media engine isn't just a collection of completely independent encoder/decoder blocks for each codec but a kind of simplified specialized microcoded CPU with a collection of fixed-function blocks which can be shared between different codecs. So it could have blocks that do RGB->YUV conversion, Discrete Cosine Transforms, etc. and you can use the same DCT block for AV1, HEVC, and AVC. Maybe you can also create specialized efficient ways to transfer frames back and forth with the GPU, for sharing cache with the GPU, etc.