I've also posted my solution on StackOverflow: https://stackoverflow.com/a/67308098/15788389. Hopefully one of these links spans the ages.

You're saying "sidecar" but I think you're describing what I would call "sideload". If that's true, I've succeeded at doing this, and I've dropped an explanation and my source code on StackOverflow: https://stackoverflow.com/a/67308098/15788389 and on github: https://github.com/kanderson-wellbeats/sideloadWebVttToAVPlayer. I apologize for linking out instead of posting stuff here directly but Apple does not let me post my full answer because it's too long.

I got this working. It was a horrible experience but I did succeed. My full explanation is too long to post here so I've stuck my long explanation and sample code on Github: https://github.com/kanderson-wellbeats/sideloadWebVttToAVPlayer and on StackOverflow: https://stackoverflow.com/a/67308098/15788389 .

I got this working. It was a horrible experience but I did succeed. My full explanation is too long to post here so I've stuck my long explanation and sample code on Github: https://github.com/kanderson-wellbeats/sideloadWebVttToAVPlayer . I dearly hope my link doesn't die. Apple doesn't want us posting long answers to questions but they're just fine forcing us to do weeks of ridiculous work for something that on another platform is an hour or less.

I got this working. It was a horrible experience but I did succeed. My full explanation is too long to post here so I've stuck my long explanation and sample code on Github: https://github.com/kanderson-wellbeats/sideloadWebVttToAVPlayer . I dearly hope my link doesn't die. Apple doesn't want us posting long answers to questions but they're just fine forcing us to do weeks of ridiculous work for something that on another platform is an hour or less.

The github links I posted earlier turned out to not be as helpful as I thought. I'd like to remove those posts but I can't. All in all this was a horrible experience but I did succeed. My full explanation is too long to post here so I've stuck my long explanation and sample code on Github: https://github.com/kanderson-wellbeats/sideloadWebVttToAVPlayer . I dearly hope my link doesn't die. Apple doesn't want us posting long answers to questions but they're just fine forcing us to do weeks of ridiculous work for something that on another platform is an hour or less.

actually this is going very badly. avoid this if you can. i can't get it to work in new and exciting ways nobody else mentioned.

i'm another poor soul going down this road, and i haven't yet gotten this to actually work, but i wanted to remedy the dead link above - jbweimar's site is down, but the code is here: https://github.com/jbweimar/external-webvtt-example another code sample is here: https://github.com/jaredsinclair/sodes-audio-example which has an accompanying blog post with helpful explanation (blog post is linked from the github post) this comment is also helpful: https://github.com/google/shaka-packager/issues/575#issuecomment-534471091