To give an example, I've seen some multiplayer games with dynamic content, that use Websockets for communication with the server and update various information via data URIs. I've never seen a text/html data URI yet (mostly image transmission to be honest) but for a multi-client Websockets type application I definitely wouldn't rule out that sort of thing.

I agree that blocking the rendering of data:text/html (and any other MIME type that could be used maliciously) from the address bar is a good idea. I can't think of a valid use case for that scenario. It seems like similar attack vectors have been known for some time (https://nakedsecurity.sophos.com/2012/08/31/phishing-without...).