It's definitely still possible since effectively the cast device just runs a (display-less, in case of speakers) browser instance navigating to some URL and communicating with the remote using a message passing API, right? You can display whatever you want in that URL, including HTML media elements playing back HLS or MPEG-DASH streams.

I believe they used to have some default receiver app implementations that you could just send a media URL and they'd play that back; is that what they removed?