If the vendor locks the bootloader tightly, or uses a chipset that does so, then a custom ROM isn't possible. VoLTE also has ended the lives of many devices as

If the vendor locks the bootloader tightly, or uses a chipset that does so, then a custom ROM isn't possible. VoLTE also has ended the lives of many devices as 3g networks have shut down. Samsung's VoLTE isn't supported in Lineage, so that vendor is gone. There could be a few reasons.