How do I hide the top-bar in Roblox?

To disable the topbar, use the following code in a LocalScript under StarterPlayerScripts:

wait()
pcall(function()
	local starterGui = game:GetService('StarterGui')
	starterGui:SetCore("TopbarEnabled", false)
end)

Technical Details

A pcall is necessary because the feature is behind a flag. If we have to roll back, the SetCore call would error, but the pcall would allow your code to continue without halting.

GUI spaces' origin is still at 0,36 from the viewport render.

The topmost 36 pixels of the game will not trigger gui events such as button presses.