The bad news is that the Fortnite Playground limited time mode that was rolled out a couple of days ago and then very quickly taken down is still down. Epic said in its most recent update on Reddit that it is continuing to test required improvements, and promised to post another update "shortly." Of course, that's basically what it said yesterday and the day before, so as timelines go it's not the most carved-in-stone you'll ever encounter.
But there is good news, too.
We worked out those issues and 50v50 is now enabled. Get out there!June 29, 2018
The 50v50 mode was meant to be enabled this morning to take the place of the Final Fight LTM, which "isn't quite in a state where we want it to be" just yet. But that plan fell apart too, when Epic "discovered a few issues" with the mode and delayed the rollout. Hopefully it will stick this time, but we'll let you know if not. (And when the Playground is back, too.)