I downloaded all the latest (9.1.0) addons of badboy with wowup and only BadBoy History appears in the Interface menu.
I even disabled ALL other addons and it still does nothing (+it's definitely set to load +tried only with the core addon).

Is there a way to debug this? I'm not unfamiliar with addon usage (or even basic authoring) in this game. This is very odd behavior to me.

As described in the project description on the project page, the command to open the options is /badboy

As I had said, "only BadBoy History appears in the Interface menu." which was different than the expected behavior I was seeing from the docs of the addon.
But it's not important to me now because after the ban of some of the spammers I don't use it anymore.

The "docs of the addon" (project page description?) do not mention it being in the interface options so I have no clue how you came to the conclusion that this was "expected behavior"?
Regardless, like I said, the command is /badboy.