/usr/include/plymouth-1/ply-splash-core/ is in libplymouth-dev 0.9.2-4.
This file is owned by root:root, with mode 0o755.
..
/usr/include/plymouth-1/ply-splash-core/ply-boot-splash-plugin.h
/usr/include/plymouth-1/ply-splash-core/ply-boot-splash.h
/usr/include/plymouth-1/ply-splash-core/ply-device-manager.h
/usr/include/plymouth-1/ply-splash-core/ply-keyboard.h
/usr/include/plymouth-1/ply-splash-core/ply-pixel-buffer.h
/usr/include/plymouth-1/ply-splash-core/ply-pixel-display.h
/usr/include/plymouth-1/ply-splash-core/ply-renderer-plugin.h
/usr/include/plymouth-1/ply-splash-core/ply-renderer.h
/usr/include/plymouth-1/ply-splash-core/ply-seat.h
/usr/include/plymouth-1/ply-splash-core/ply-terminal.h
/usr/include/plymouth-1/ply-splash-core/ply-text-display.h
/usr/include/plymouth-1/ply-splash-core/ply-text-progress-bar.h
/usr/include/plymouth-1/ply-splash-core/ply-text-step-bar.h