Arch includes nothing … they make packages available, add a wiki entry, and you install & configure it yourself.
SELinux is not “officially” supported by Arch, the userspace tools and profiles are in the AUR, but their kernels make it available … so it is technically possible if you are willing to do the work.
The whole point of EndeavourOS is a simple, minimal, vanilla Arch install for users to then customize to their liking.
An EndOS install is the not an OOTB end point, it is an ubloated starting point, a first step in creating your system.
The more minimal the starting point the better IMHO.