lineageOS, previously CyanogenMod, is the oldest name in the custom ROM community. It is the most popular aftermarket Android distribution followed by other AOSP-based ROMs. Most of the custom ROMs for Android devices are based on LineageOS. Hence, it plays an important role and any minor change or bug can affect almost all the custom ROMs out there. LineageOS comes with its own optional su binary addon to achieve root access on the ROM. Unfortunately, this will be not included going forward with LineageOS 17 as the project is dropping its PrivacyGuard feature in favour of Google‘s Permissions Hub.
Long before Gooogle added granular controls for app permissions in AOSP, LineageOS has been offering a similar set of features(even better) known as PrivacyGuard. But as the developers behind the project could not port it to Android 10, they have decided to drop it. Instead, they will adopt Permission Hub as it offers a similar set of features. Since su addon depends on PrivacyGuard, it will be not offered for LineageOS 17 which is slated to release soon.
However, root access over ADB will be supported but if the user wants to grant root access to apps, they will have to install none other than Magisk which is currently the most popular root method.