Android Verified Boot (AVB) was present on some phones with Oreo. Google have implemented it to protect against outdated security patches. The company will also impose it on new phones with Pie out of the box. AVB may also make non-stock firmware building more difficult.
https://www.notebookcheck.net/You-won-t-be-able-to-downgrade-a-new-Android-that-ships-with-Pie.321076.0.html