Flying Buttress is described as 'As Apple's built-in Firewall preferences improves in both functionality and security, I will be aiming Flying Buttress more at users who need the advanced firewall configuration, logging, and IP sharing options found in Flying Buttress' and is a Firewall in the security & privacy category. There are three alternatives to Flying Buttress for Mac. The best alternative is Little Snitch. Other great apps like Flying Buttress are Radio Silence and Vallum.
Flying Buttress alternatives are mainly Firewalls but may also be Network Monitors. Filter by these if you want a narrower list of alternatives or looking for a specific functionality of Flying Buttress.