/usr/lib/x86_64-linux-gnu/X11/rstart/commands/x is in x11-session-utils 7.7+2.
This file is owned by root:root, with mode 0o777.
It is a symlink to /usr/lib/x86_64-linux-gnu/X11/rstart/commands/x11r6
/usr/lib/x86_64-linux-gnu/X11/rstart/commands/x11r6