alt Hacker NewsCompatibility will always be the hard part with new standards like this. Given that most terminals already have some special handling of box drawing characters, I don't know if it's completely dead on arrival for implementors, but I don't love as an application author with ratatui, not knowing how the padding on my app will look. I might honestly rather disable this and know it'll be more consistent, than enable it and fragment my UI.
The ratatui change was a pure PoC, isolated branch, my own fork, not the ratatui repo even. This would likely never ship as a default approach as it would be way too breaking. I haven't really spent any time on making it hook into the current block border approaches - it's just at the cell level where this is opted in. That will never merge without much much more though into things.
I used ratatui there mostly so that I could prove that it would work in real looking apps that do panels and similar stuff that benefits from borders.
But yeah, compat is key. It would probably go out to ghostty, rio, kitty pretty easily, terminal.app never, other terminals much slower. IDK about the windows / linux side of things.