Not exactly DM agency (or I wouldn't call it this) but one of the players started out writing his character with a murder-hate-on for dragonborn, which exist in my setting. I asked him to please pick a non-PC race, so he wasn't barring another player from playing a dragonborn. He moderated it, to a strong dislike.
Maybe a better of DM agency would be disallowing PC races. I don't allow drow or duergar or even deep gnomes, at least in part because they were exterminated (for the most part) for reasons.
Maybe choosing not to permit PCs to change things in the setting that are there because I want them there. My setting has no gods. I know what happened, and I know how it could be undone (sort of, it wouldn't bring the previous gods back so much as enable new beings to become gods) but I really don't want PCs to do it, so they might find out about the how, but I have zero intention of allowing them to find the necessary tools.
Maybe a better of DM agency would be disallowing PC races. I don't allow drow or duergar or even deep gnomes, at least in part because they were exterminated (for the most part) for reasons.
Maybe choosing not to permit PCs to change things in the setting that are there because I want them there. My setting has no gods. I know what happened, and I know how it could be undone (sort of, it wouldn't bring the previous gods back so much as enable new beings to become gods) but I really don't want PCs to do it, so they might find out about the how, but I have zero intention of allowing them to find the necessary tools.