With the release of iOS & iPadOS 16, Apple has introduced a new "Developer Mode" option for users which you may have encountered by now if you've seen this popup.
What is Developer Mode?
"Developer Mode" basically means that you are by default prohibited from downloading Apps/IPAs outside the App Store. So sideloading & installing apps outside of Apple's walled garden is definitely a no-no from Apple.
While sideloading does carry some risk, the potential harm is severely limited because iOS sandboxes each app, preventing it from affecting your entire device—especially if you’re running the latest iOS version.
With that being said, this does not mean you should scour the internet to sideload/install every .IPA you can find. You should still stick to official IPA sources which are known to be trustworthy—such as iOSGods.com.
How do I enable Developer Mode so I can install Third Party Apps?
STEP 1: Open Settings on your iPhone or iPad.
STEP 2: Tap on Privacy & Security option in Settings.
STEP 3: Scroll down to the Developer Mode option and navigate into it.
STEP 4: Tap the switch to enable Developer Mode. After you do so, Settings presents an alert to warn you that Developer Mode reduces the security of your device. To continue enabling Developer Mode, tap the alert’s Restart button.
STEP 5: After the device restarts and you unlock it, the device shows an alert confirming that you want to enable Developer Mode. Tap Turn On, and enter your device passcode when prompted.
How do I disable Developer Mode?
If you want to disable Developer Mode, you do so directly on the device. Go to Settings > Privacy & Security > Developer Mode to show the Developer Mode toggle switch.
Like enabling, disabling Developer Mode requires a device reboot before it becomes effective. After you disable Developer Mode, you can’t run apps from Xcode or Sideloadly on the device until you re-enable it, using the same switch in your device’s Settings.
Please note that Developer Mode must be kept ON for your sideloaded apps to continue working. If you do not see the option, try sideloading an app via Sideloadly as that’s been known to help.
Thanks to nerdschalk for the images!