Epic have confirmed over on Twitch that the Playground mode will be a permanent addition to the game in the future, not just an LTM. The following was stated from the official Fortnite account:
“Yeah, we are working on a version where we can keep Playground as a mode, not just a Limited Time Mode”
It isn’t clear whether the current iteration of Playground will be kept or if a new and improved version will take it’s place. Either way, it’s here to stay.