It seems that the Archwiki tutorial is too old, it seems that only need to modify BACKEND to make SSHGuard and firewalld work together.
That seems to be what’s going on. Unfortunately, I don’t have a Linux machine with firewalld on which I can test my changes.
Would you mind putting together a patch against sshg-fw-firewalld that works for you, that I can review and commit?
Do you also happen to know if the new syntax is backwards compatible with older versions of firewalld?
It seems that firewalld has indeed changed some commands, and some distributions do use the old firewalld. Because I use a rolling distribution, I am not particularly clear about how the old firewalld is used, and I started using sshguard in the past week, so It might be better to ask the developers of firewalld directly.