It appears Twitter wasn't keen on the idea of people being able to play retro games from the Internet Archive's MS-DOS repository from inside tweets and has since disabled the unintended "feature".
While Twitter does provide an API for developers to access and use its services, it has yet to go down the path Facebook did by providing a framework to create rich applications hosted within its social networking site. So when something like this comes along — completely by accident — the clamp down is not surprising.
According to VentureBeat's Paul Sawers, it also happened to violate Twitter's "rules of engagement", which specifically target gaming (among other things):
Indeed, we can confirm that the reason it was switched off is because it violated Twitter’s Card Rules, which govern how developers can and can’t use rich media content within tweets.
That, and all the potential security and privacy concerns it leaves the door open to.