LineageOS is one of the most popular custom ROMs out there, bringing newer versions of Android to devices that OEMs have abandoned. Unfortunately, support does not last forever with LineageOS either, as the company has dropped support for LineageOS 16.0 and Android 9.0 Pie. As a result, 24 devices will no longer receive security patch updates, including the Huawei P20 Pro.
https://www.notebookcheck.net/LineageOS-leaves-two-dozen-devices-adrift-by-ending-Android-9-0-Pie-support.522558.0.html
Not entirely true as far as security updated. True there will be none for Android 9 but these phones can move up to android 10 or 11 and still receive security updated.