Earlier quoted context omitted.
mm, text to bitmap or vice-verse isn't there though is it? edit: plus the blu-ray subtitle encoder itself is new...
The fact that there is no newer bug ticket for it in 12 years shows is this a purely academic feature nobody wanted. And why should they? Most Blu-ray players support the old format (SRT); converting it to PGS gains almost none of the upsides of the newer format (screen placement).
certainly fades / karaoke style subtitles from Advanced SubStation Alpha on blu-ray devices aren't everyone's cup of tea.
but i do enjoy antialiased blu-ray subtitles of course YMMV :)