Back when I cared about physical media I wrote a substation alpha to blu-ray subtitle encoder.
It isn't exactly spec compliant. Hilariously, the spec is patented dispite prior art & simple run length encoding: https://patents.google.com/patent/US20090185789/da).
Output is good enough for power dvd when using the admittedly closed source TSMuxer. Always meant to try to port it to ffmpeg, but it just doesn't seem worthwhile, given the context of Netflix.