I don’t know if it’s by design or an oversight, but I just discovered a nifty little #iOS26 reality.
Overall I like the new full UI screenshot behavior, but on rare occasion the former disappearing thumbnail behavior is preferred, such as when capturing images while FaceTiming with my grandson.
Turns Out™ you can have both. Leave the sleep/up volume command to use the new behavior and set the triple tap feature on the back of your phone to do a screen capture which will use the old behavior.