'Discontinued' implies there is no plan for an improvement in the future (sometime between soon™ and soonish™). Hassan having last updated Inspector in... June 22nd, you can be assured that it is indeed outdated and therefore, will only work with Spongeforge builds around that time.
Considering that Spongeforge allows for build numbers, versioning, and has an impressive changelog, you, as a server admin, can always make an informed decision. The dates and record are there to show it. Inspector, as far as I have used, will not work with latest versions - and if it ever manages to boot up, you can rest assured it'll have a negative impact on server performance. If you wanted to know, you could always test on your own test server.
Lastly, the responsibility for using plugins rests on the user, not the provider. Hassan's plugins have served the Sponge community superbly (s/o to EssentialCmds), but an outdated plugin is an outdated plugin. And us server managers ought to know what it implies, when you are easily 2000 builds behind the latest stable Spongeforge for 1.10.2. This is not me telling you
free=happy, this is me explaining that most of the time, with any modded community, chosing the plugin to use rests on the user. Give priority to plugins with active developers, with a responsive thread, and it'll reward you much more than using a half-year old plugin (even when this fact is clearly indicated).