Universal Control won't be available when iPadOS 15 launches on Monday, though Apple still promises a fall release window.
An update to the iPadOS 15 page on Apple's website shows Universal Control won't be available until later in the fall. The feature was teased at WWDC, but it never officially showed up in any of the beta tests for iPadOS 15 or macOS Monterey.
The feature requires a Mac running macOS Monterey to initiate cursor sharing, so the later release likely coincides with the macOS Monterey release window. Apple hasn't said when macOS Monterey will release beyond "this fall."
The Universal Control feature will enable a controlling Mac to pass its cursor between an iPad, the controlling Mac, and another Mac. The WWDC demo showed the user move a file across multiple device's desktops onto the controlling Mac's desktop.
Apple is expected to hold a Mac-focused event in the coming weeks to announce its pro-grade Macs with the "M1X" processor. The company will likely time the macOS Monterey release with that event.