Plymouth with disk encryption not showing splash at startup

If your boot partition is encrypted (as part of a full disk encrypted setup), at this early stage in the boot process, there is only grub to unlock the drive to get to the initramfs and kernel. So AFAIK, no, there’s no way for plymouth to inject itself before the actual filesystem is unlocked and therefore no way around grub’s textual password prompt.

However, in case of an unencrypted /boot, things may be different.