Hi @Jac and possibly @HainjeDAF, I moved this post here where it belongs.
My understanding of ITTS is much more solid (ITTS (DCC text) reimplementation) than the extraction from S/PDIF.
The STM32F746NG can do it and it is called user data in the documentation.
The chip itself is sold out as you might have guessed @Jac, but the board I am using (STM32F769I-DISCO) is almost reasonably available and I bought mine in the end of 2019. It is perfect with a touchscreen, coaxial S/PDIF in and out, Ethernet, all the peripherals I could dream about apart from a built-in read-amplifier 
.
I am too incompetent to understand how to work with STM32 DMA or interrupts, though. If anyone with a working 1st or 2nd gen DCC recorder could help me with that, I would sent over my ITTS tape, my board and port ITTS.js to whatever target necessary.
The Teensy 4.0 and its underlying chip do not seem to support sub-channel data natively.
@Jac If I understood your latest hackaday post correctly, you can access it but do not have a way to properly push it to a PC? I could certainly waste my Glasgow FPGA board on it when it arrives, but this is far too overkill imo.
And in general I should stop procrastinating getting my pile of parts from three 2nd gen recorders and the one with a broken gear and missing parts to @Jorn for repair. Thank you for that in advance 
!