macOS 10.15 (and higher) will be receiving a version of Managed Software Center that will not install some macOS updates, but will instead send users to the Software Update preference pane in System Preferences.
Generally, this is limited to software updates that require a restart. Updates that don’t require an update will still be installed by MSC; if there is a blend of updates that require a restart and those that do not, MSC will still defer to Software Update.
Apple says we will be able to “hide” the macOS Upgrades (eg, 10.15 to 10.16) banner from Software Update for a short time, about 60 days. Additionally, we expect all future macOS Upgrades to be driven through Software Update, and our ability to “push” them will be limited or not possible.
Going forward, setting macOS 10.16 to “Install” in Izzy will likely not work; it will likely require users to manually click through Software Update.