I couldn’t find it for free but under USD 17 is good enough. The Mitsubishi M51581 seems to be present in all ITTS compatible 1st/2nd gen DCC recorders. The Philips TDA1315 in the 3rd gen players should support it as well, but it is stored in buffers to be accessed via registers, not streamed out. Same story as in many more modern S/PDIF receivers.
I fully agree that we shouldnt replicate it. But googling I found an interesting application note by ST Micro from 2018. The STM32F4/F7/H7 ARM microcontrollers are able to transmit receive S/PDIF including user data and have lots of IO options like USB, Ethernet, Micro SD, Multitouch Touchscreen and HDMI. A great value dev kit is avaible for under USD 90, what are your thoughts on it for a first look on the specifications? Model number: STM32F769I-DISCO