This update allows users to block or mark content or blogs that they consider sensitive. Under-18-year-old users cannot access these sensitive content anymore as they cannot turn the "safe mode" off.
However, just after its launch, users noticed that LGBT content was blocked by mistake but that pornographic content was still accessible.
They have mobilized to try to find a way to hack the system and unlock this "safe mode". They even launched a petition calling for the removal of this security filter.
Since then, Tumblr staff apologized for the filtering of LGBT posts indicating that the so-called problem had come from users who had mass-blocked LGBT content:
"We’ve heard from a bunch of you that Safe Mode was filtering posts from the LGBTQ+ community even though they were completely innocuous and totally safe-for-work. Please know that was never our intention, and we appreciate you letting us know so quickly—and forcefully! We’re deeply sorry. Tumblr will always be a place where everyone is welcome and protected, so we want to explain what happened.
The major issue was some Tumblrs had marked themselves as Adult/NSFW (now Explicit) as a courtesy to their fellow users, and their perfectly safe posts were getting marked sensitive unintentionally. That should never have happened. We’re sorry."
In short, the team is working on improving the filter.
It's strange to always see the LGBT content blocked when a system like this is installed.