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Normally when an OEM complies with Google's requirements for GApps and pre-loads them on their devices, the devices have been certified for the apps to ensure everything works as Google intended. The problem is that some smaller OEMs were skipping that certification process, yet the OEMs were installing and shipping the GApps on their devices. Some OEMs were shipping devices minus the apps but giving users instructions on how to sideload them.
Both of those tactics to get GApps were loopholes that Google has now patched. Google is checking the build date of a system image and if the device is uncertified and the build image was created after March 16, 2018, Google apps will no longer work on the device. This isn’t a surprise to device makers or owners of non-certified devices. Google has been warning for a year that this block was coming.
End users of non-certified devices have been getting warnings when booting their devices for at least a year noting that the device isn't certified by Google. Until now, the users of the devices have been able to clear data on Play Services and continue using the GApps. That workaround is no longer an option.
If you are the sort that likes to run a custom ROM on your device, you may be wondering what that means for you. Google is aware of custom ROM users and has a whitelist for users to submit their Android ID to allow GApps to run on that particular ROM. Google does have a limit of 100 devices per user. You can get your Android ID using the ADB command "settings get secure android_id" and once obtained, you can insert it on Google's site to get on the whitelist. A caveat is that the device ID changes with each factory reset, so you can only do that 100 times before you are ineligible for the whitelist.