You claim that the trivia doesn't pertain to "in-game animals, and so it’s not relevant here." Yet, you've kept this trivia: "The fictional Wapiti tribe is not based....late nineteenth century." If it doesn't pertain to in-game content, then why is that allowed, especially when it's speculative and the language is not neutral?
In regards to the overhunting about bison and theories as to how/why their numbers dwindled, the general nature of this topic means that it’s not particularly relevant for a wiki devoted to Red Dead. Many things are explicitly mentioned in the games, like rapidly advancing technology, ever-increasing government interference and so on, yet not all of these would need theories on the real-life factors behind them. It’s simply not relevant.
The point about the inspiration for the Wapiti Indians provides a potentially insightful comparison between them and the real-life Indians in similar situations, so there is a reason for that. However, going further and discussing reasons behind it and whatnot would not be necessary, hence why the point is relatively broad and vague. Similarly, the buffalo page could plausibly have a brief mention of how the real-life buffalo’s numbers were low by the time of the game(s) for comparison, but going further and discussing the reasons behind it (especially when it’s theoretical) is a step too far and no longer pertains to anything in-game.
Regarding neutral language, it’s certainly true that wording should be as objective as possible on articles. However, I wouldn’t say there’s much wrong with the current wording.