A notice in “Other changes” in the 11.20.0 Release Notes mentions
Apple deprecated the following keys in macOS 26*, iOS 26*, iPadOS 26*, and tvOS 26*. This change deprecates the Defer updates of settings in the Restrictions payload for computer configuration profiles and the Defer software update setting in the Restrictions payload for mobile device configuration profiles.
This is not currently noted in the Restrictions profiles for devices or computers. They mention using Blueprint builder to manage deferrals going forward. Time to test Blueprints once we get past the last deferrals or 26 apparently.