Apparently tweeting the 996.icu website from the official Twitter Android/iOS clients causes it to refuse to send at all. You don't even need to be in China to see the result, because it blacklists the link globally. It allows the link on web and 3rd party clients, though.[1]
Edit: if this is a glitch from trying to combat bots Tweeting the URL, that just means you can trigger automatic censorship of any URL with a bot farm.
This is very unlikely because it seems to be something that happens in the official client rather than server-side. Spammers are unlikely to use official clients and blocking them there doesn't make any sense.
Having said that, if they were intentionally trying to block this then using their anti-spam mechanisms would be an obvious way of doing so, so I think it's definitely a possibility that this is some accidental side-effect of something else rather than a simple intentional block.
An anti spam feature should not links typed by real humans on purpose. Lots of popular links are shared on twitter and don't get blocked via anti spam. This seems like something deliberate, even if the global part of the impact is a deployment error.
[1] https://twitter.com/copiesofcopies/status/111403968059961344...