One player write-up, plus the route list
Jeff the Killer route
Jeff is on the official six. The steps below come from one detailed itch comment. I am saying that up front so a single memory is not dressed up as a developer guide. A second comment agrees that the knife in his room opens other doors.
Spoilers: Chapter 1, as that comment tells it
Starlightmidnight's comment says Jeff must be your favorite and like you enough; it does not give an affection threshold or a way to calculate it. The player describes accepting his room offer after meeting everyone, a weapons closet, going outside with Ben, meeting Eyeless Jack, and choosing Jeff or Ben. The same player says to refuse the proxy invitation. Single source: these are reported scene notes, not a guaranteed route recipe. No weapon options or preferred weapon are named.
The developer listing names Jeff as a route (single source). The knife discussion concerns opening doors; it does not verify the closet sequence or an ending condition.
Spoilers: Chapter 4 notes from players
blueberrypieo reports being frightened by a Jeff scene. Separately, NerdyB1443 reports dialogue repeating before the proxy task. These are different claims, not two independent confirmations of one bug. Neither provides a workaround or evidence that a fix has shipped.
Questions
Do I have to refuse the proxy task?
The same comment that describes the closet says yes, refuse Slenderman's proxy invite to stay on Jeff. I have not found a second written source.
Which weapon?
The comment says you choose a weapon. It does not name the options or say they change the ending.
Is Jeff the default route?
The official list is not ranked. I am not calling any route the default.