FreeBSD has the concept of a 'security level'. You can increase it at runtime, which disables more functionality, but you can't decrease it without a reboot.
At security level 1, the immutable and append only attributes on files can't be removed, so even chattr -i would be useless.
It goes far beyond making files immutable. I haven't really done a deep dive to see if it's on par with SELinux but the description in this thread doesn't do it justice.
What I meant is that devising a sane and useful way to make use of security levels seems easier than achieving something 'equivalent' with SELinux. Sophisticated policy systems are nice, but something that kind of bundles sane defaults together and organizes them into ordered layers like security levels sounds great.
At security level 1, the immutable and append only attributes on files can't be removed, so even chattr -i would be useless.