Google is improving security in its system, to avoid unwanted accesses or hacks that change the firmware itself with no permission. At the moment only reserved to its Pixels.
After receiving the latest update that includes the May security patches, Google Pixel and Pixel XL owners will realize that if they were rooted before, now they no longer have access to it anymore. The reason? This is explained by a great expert, the best in this aspect, the man who started the rooting war: Chainfire.
The reason for this problem lies in the changes made by Big G in its new updates: the booloader included in the latest firmware, no longer accepts the image (boot image) without a sign. It seems that Google is using AVB (Android Verified Boot) to verify that boot images are signed before the flash, if it is not, you cannot flash anything. Although the image does not have to be signed using an OEM key; You can use a custom key.
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Chainfire, in a thread on XDA, explained all the steps to follow to generate your own custom signature keys so that your device will keep root permissions enabled with May patches and also future ones.
Since the issue is not very straightforward and requires some knowledge of the topic, here we are not explaining the whole procedure: if you have a Google Pixel or Pixel XL and you are interested, you can follow the link down here, referring directly to an XDA post.
We are not responsible of anything that could go wrong to your device, so take your responsabilities before proceeding!