This thread is completed, and discussion about the Official release should be held here:
And no, it is not supported to lock the bootloader on official builds, as they use publicly available keys for androids verified boot. Even thought it would be possible to re-lock using them, it makes no sense since everyone could just sign the images again. If you want to re-lock the bootloader you must build LineageOS yourself and sign it with your own keys: Locking the bootloader · ArianK16a/android_device_fairphone_FP6 Wiki · GitHub