One approach to rule them all?
I was just wondering, in the terms used by LFS201, if "modern" is to be read as "whatever Red Hat prefer at the moment", and "legacy" means "used by other distribution families"?
The Firewalls chapter seems -- to this user of Debian derivatives at least -- to be all about skipping over other approaches and zeroing in on the RH/systemd approach as embodied by firewalld and firewall-cmd. More or less to the exclusion of all others, including the "legacy" iptables. I'm not sure whether "drop what you're doing and install Red Hat's alternative" is as educational as it is indoctrinaire.
I appreciate that space is limited, and that you can't cover every last angle. But perhaps these limitations can be met without glossing over or ignoring non-RH practices. A great deal of the examples seem to start with the RH way of doing things first, and others later if at all.