Killers farming or playing just to complete a challenge
If you want to farm or play a match solely to complete a challenge that's your prerogative. However, I have exactly zero interest in taking part in those types of matches. So if that is what you plan to do at least send a message about it while still in the lobby. If I get your message while still in the lobby I will happily leave the lobby and let you find someone that is interested in taking part.
The first 5 matches I played today I received messages either while the match was loading or after it started from killers wanting to farm or wanting the survivors to help them complete a challenge.
If I get a message about it while the game is loading I'll just drop out and go back to the lobby. I do realize that means everyone also gets the match cancelled. If I get a message about it after the game has already started I will likely just DC. If my DC means you can't complete your challenge I don't care. The challenges weren't meant to be teamwork anyway. If the penalties are in place I will happily take a 5 minute penalty as opposed to running around for 15 minutes taking turns being downed and healed or whatever else is trying to be accomplished.
The bottom line is if you wait until after the lobby to tell me you plan to do one of those things I am forced to take part in a nonmatch if I stick around. If you have no problem not giving me a choice as to whether I want to take part in such a match I have no problem pulling out of it.