What is the half-open perimeter anti-pattern, and why is it a bad idea today?

The whole firewall rule set rests on "inside is all good, outside is all bad": every connection from inside to outside is allowed (company -> internet: allow any), every connection from outside in is denied (internet -> company: deny all). It assumes every actor fully controls all its actions, which no longer holds.

In the early internet, networking was a matter for a few experts and IT was simpler, so the assumption may have been closer to true. Today it fails in exactly the outbound direction it leaves open: malware on an internal machine, a phishing victim, or an insider can send anything anywhere, and the perimeter does nothing to stop data leaving. So it should never be the only means of structuring a company network; it gives up the "break-out protection" a perimeter could provide.