You won't be able to downgrade a new Android that ships with Pie
There are people who end up preferring older versions of Android over new releases such as Android Pie. Some of these people will prefer to roll back updates, even in the face of losing newer security patches. It now appears that Google wants to make this more difficult. The XDA contributor Mishaal Rahman has reported that new phones that will ship with Pie installed will also have Android Verified Boot 2.0 (or AVB 2.0).
Earlier versions of AVB have been integrated into some Oreo-based builds. It functions to prevent a phone from booting with an older rollback image. In other words, someone with a phone running 8.1.0 with AVB will not be able to install 8.0 again. Google has imposed AVB 2.0 on all devices that launch with 9.0.
Rahman observed that AVB 2.0 affects partitions such as boot, vendor and system. Therefore, developers may have to disable Verified Boot when building their next custom ROMs, if they want them based on Pie. Rahman encountered this himself while working on his Oneplus 6. His report also covers fears from LineageOS team member npjohnson that smaller phone companies may be affected by the increased costs involved in developing new types of eXtensible bootloader (XBL). This is just one example of an adaptation that may have to be made in order to match non-AVB-affected partitions (e.g. modem) to those needed by Android.
However, AVB 2.0 may not be all bad news. Google may have implemented it in order to address CVE vulnerabilities. In addition, it is just possible that AVB may enable custom images to work without unlocking a bootloader. We will have to wait for the release of phones such as the Pixel 3 XL to know for sure.