This warning applies specifically to links and not to the content of the tweet itself. If those links lead to an image or video, that image or video has to have some kind of warning on it for Twitter to flag it.
If you don’t want to see the warning when this new feature goes live, you can always go to your Twitter settings and on the Account tab you can check a box that says “Display media that may contain sensitive content.” Also, if you post sensitive content, you can flag your account as such on the same tab so you don’t run afoul of Twitter’s Media Policy.
Expect an announcement soon when testing is done and the new code is officially launched.
[Image credit: Pete Simon]