And in 2010, a similar backdoor appeared in UnrealICRD: https://lwn.net/Articles/392201/. Also in proftpd: https://www.aldeid.com/wiki/Exploits/proftpd-1.3.3c-backdoor. Both were done by ac1db1tch3z who the author of OP's post, Ben Hawkes, got a shoutout from for another local privilege escalation vulnerability from over a decade ago :-).

Anyways, in response to the backdoor in unrealircd, Core Security came up with a "hiding backdoors in plain sight" challenge: https://seclists.org/fulldisclosure/2010/Jul/66

"Bugdoors" are not new, and I'm sure some have been patched without anybody realizing they were introduced maliciously.