Top comment in that thread is an expression of hesitation over the pricing. Looks like they kept largely the same scheme over their lifetime, except by replacing the next step up from the $99 tier -- $299 for 50 servers -- with a $499 for 75 servers:

http://web.archive.org/web/20151205162125/https://appcanary....

http://web.archive.org/web/20171026173951/https://appcanary....

I like the idea of smartly compiling and packaging vulnerabilities and building a business off of that. But was the pricing competitive to what a company devops employee would typically spend in monitoring this? It looks like they provided monitoring and reports with these plans but not anything else on top of sending the alerts.