I saw that a colleague of yours has already claimed that “this has already been fixed”, but it’s very far from being fixed in my opinion.
Here’s the original thread:
and well, the option defaultTextTrack has no description and no usage in the docs.
I am a web interface user - on the web interface, there’s only an option to add captions. There is no option to set them to auto-play and they are off by default for the viewers of the video when embedded.