I believe the app stays available in the Purchases section like you said if the developer "unpublishes" the app, but not if the account or app is "terminated" by Apple.
In addition to Dash (which I'm not able to locate in my Purchases tab), I've noticed this to be the case with apps that violate Apple's rules, like those hidden proxies or emulators that have sprung up over the years and were later removed by Apple.
In addition to Dash (which I'm not able to locate in my Purchases tab), I've noticed this to be the case with apps that violate Apple's rules, like those hidden proxies or emulators that have sprung up over the years and were later removed by Apple.