SELinux can be disabled at boot time by an argument in
/etc/default/grub.
Remove any instances of selinux=0 from the kernel arguments in that
file to prevent SELinux from being disabled at boot.
Rationale
Disabling a major host protection feature, such as SELinux, at boot time prevents
it from confining system services at boot time. Further, it increases
the chances that it will remain off during system operation.
ISA-62443-2-1-2009, Security for Industrial Automation and Control Systems Part 2-1: Establishing an Industrial Automation and Control Systems Security Program