After installing 16.04 without third-party drivers and with Secure Boot still enabled, I clicked on Additional Drivers in Software & Updates settings, and selected the one available driver, microcode firmware for Intel CPUs. After rebooting, the setting still shows "1 proprietary driver in use".
The Ubuntu Software tab in Software & Updates has "restricted" set by default (as well as main, universe and multiverse).
After installing 16.04 without third-party drivers and with Secure Boot still enabled, I clicked on Additional Drivers in Software & Updates settings, and selected the one available driver, microcode firmware for Intel CPUs. After rebooting, the setting still shows "1 proprietary driver in use".
The Ubuntu Software tab in Software & Updates has "restricted" set by default (as well as main, universe and multiverse).
So I'm not sure what you say is entirely true.