It's sort of weird that the successor to the ultimate sandbox, Garry's Mod, which is called s&box and pronounced sandbox, apparently wasn't going to have an actual sandbox mode. But the game development platform from Facepunch Studios, which was announced way back in 2017, finally has a page on Steam, and in its first news post the devs say, yes, it will indeed have a sandbox mode.
"We realise that people expect an official sandbox mode from Facepunch on the engine, and have been weary about doing that," says Facepunch in the news post. "But the more we thought about it, the more we thought about a few new features that justify making it—so we're currently working on that, and won't release s&box until that's done."
