OK, I decided not to wait next Insider build upgrade to get screenshots and this thing cleared. What I did instead was the fastest way I could think of getting the screenshots: I installed an older Windows 10 PRO Insider Preview (I used build 14926) on a test machine, opted in to Fast Ring upgrades, and waited until Windows Update found the upgrade.
When upgrade files had been downloaded and prepared for the upgrade process, the Restart now button was shown in Settings. Using Restart options I could have postponed an automatic restart almost a week (system allows you to postpone it to the end of the sixth day after upgrade has been downloaded and prepared):
Regardless if I use Restart options or not, now I have the option to select either Update and Shut down / Restart, or Shut down / Restart without update (upgrade), whatever method I use to show shut down options.
In three first screenshots below #1 = Windows version and edition, #2 = showing that a new Insider build has been downloaded and prepared, #3 = shut down options. As #1 & #2 can't be shown in sign-in screen, the fourth screenshot from the same machine only shows shut down options.
I used build upgrade now to get the screenshots but it is by far not the only update / upgrade offering these options. Just remember, all critical updates need to be installed immediately, therefore option to shut down or restart without update is not always shown.
When Windows decides it's no longer possible to postpone the update / upgrade, option to shut down or restart without updating / upgrading is no longer shown