Disable screenshot preview iPad 11th gen iOS 18

The ScreenShot preview setting can be found here:

Settings > General > Screen Capture > Full-Screen Preview - set to ON or OFF as required

For information, should you ever have difficulty finding a specific feature setting with the Settings App, you can use the Search function located at the top of the Settings sidebar...

To find a setting, simply type a search clue (e.g. screenshot) into the search field. As you type, you'll see a list of candidates appear bellow it. Select a possible candidate from the list - which will automatically open the associated setting(s):

You are quite correct. The instructions that I provided do indeed relate to iPadOS 26.

While the Search function does work as described for iPadOS 18, you'll not find settings that are not available for that version of iPadOS.

Updating your iPad 11 to iPadOS 26.x is strongly recommended - as this major version includes significant features and essential security updates. For iPad devices capable of update to iPadOS 26, such as your iPad 11, security updates are now only available if updated to iPadOS 26 - at the time of writing, iPadOS 26.5 being current.