A brief history of launching Mac OS X/macOS apps…
Mac OS X 10.7 and earlier: Launch whatever app you want, the OS doesn’t care.
Mac OS X 10.7.5: Gatekeeper appears, but is a benign master, defaulting to allowing apps from anywhere. You can still install and run anything without any intervention from the OS.
Mac OS X 10.8 through 10.11: The benign master is slightly less benign, as the default setting changed (somewhere in that timeframe) to only allowing apps from the Mac App Store and registered developers. You could still disable Gatekeeper completely, though, as the “Anywhere” button was still present. If you didn’t do that and tried to launch an app from outside the store or a non-registered developer, you had to click OK in one dialog box. Still not awful, but you were aware you were working outside the Gatekeeper’s happy zone.
macOS Sierra (10.12): The benign master is now clearly just the master—the “Anywhere” button is gone. (Gatekeeper can still be disabled in Terminal, if you wish: sudo spctl --master-disable.)
And when you try to run an app from an unidentified developer, you really have to jump through some hoops…
Step 1: Double-click the downloaded app
Click OK, and you return to Finder.
Step 2: Open System Preferences
Step 3: Click into the Security & Privacy panel
Step 4: Click the Open Anyway button next to the warning about the blocked app
You’d think that’d be it, right, because you clicked a button that said Open Anyway? Nope, you’d think wrong—there’s one more step.
Step 5: Click the final Open button
Even here, the OS is trying really hard to protect you—the default is Cancel, so you can’t just hit Return.
Based on the escalation of difficulty in launching apps from unknown developers, I expect that macOS 10.13 will require a fingerprint verification, a one-pint deposit of blood with the Red Cross, and holding down seventeen keys at once while double-clicking in order to launch such apps.
Or maybe they’ll just be completely blocked.