All three generations of Xbox consoles will be able to be System Linked for LAN parties with Backwards Compatibility
Backwards compatibility is wonderful.
Backwards compatibility is a beautiful thing, especially if you grew up on gaming in the early 2000's. LAN parties, although not nearly as common as they used to be, were the only way to play rounds of Halo 2 with up to 16 buddies under a single roof. In an interview with YouTuber iJustine, Xbox's Albert Penello revealed that the Xbox One, Xbox 360, and OG Xbox will all be able to link together for LAN Parties all thanks to Backwards Compatibility.
"You can actually System Link all three generations of consoles for LAN parties. So you can get an OG Xbox, a 360, and an Xbox One...you can actually do LAN parties with your old disc on three generations of consoles."
As of right now, the possibilities of doing LAN parties are just that, possibilities. The only OG Xbox Games that are confirmed to be coming to Backwards Compatibility are Crimson Skies and Fusion Frenzy, and of those two, only Crimson Skies would work with a LAN Party as it can support up to 16 players via System Link.
Since the OG Xbox Backwards Compatible library is confirmed to be smaller than that of the Xbox 360, I wouldn't go dusting off your old copies of Halo: Combat Evolved just yet, but take solace in knowing that the possibilities are at least there.