It is probably good that OS community are exploring this however I’m not sure the technology is ready (or will ever be maybe) and it potentially undermines the labour intensive activity of producing high quality subtitling for accessibility.
I use them quite a lot and I’ve noticed they really struggle on key things like regional/national dialects, subject specific words and situations where context would allow improvement (e.g. a word invented solely in the universe of the media). So it’s probably managing 95% accuracy which is that danger zone where its good enough that no one checks it but bad enough that it can be really confusing if you are reliant on then. If we care about accessibility we need to care about it being high quality.
According to that site you can downgrade the firmware (some people really disliked the various UI changes and the firmware is getting quite prescriptive)
You can also run your own homebrew apps so I found someone who installed KOReader which they claim is better experience than default reader especially for pdfs and also link better to personal cloud storage.
There’s also ability to use locally stored Web Application Frameworks but I’m not 100% sure what the use case would be.